Should I Get the Eurail Pass?

Jacob, I suggest you read the "Man in Seat 61" website, which should answer your questions.
Specifically his page about the pros and cons of passes: https://www.seat61.com/Railpass-and-Eurail-pass-guide.htm
And his beginners guide: https://www.seat61.com/Railpass-and-Eurail-pass-guide.htm

You need to understand which trains do and do not need reservations, generally local trains don't, long distance trains do. However it is a complicated subject, as each rail company has different rules, and some local companies do not accept Eurail Passes.

You also need to understand what the Schengen Area is, and its rules.
Map of the Schengen Area: https://www.etia.com/img_dir/european-schengen-zone.jpg
Unless you are a citizen of the Schengen Area/EU, you are limited to "90 days in any 180" in the Schengen Area. You need to make sure you keep under this limit.

Some things to keep in mind: You may need to fly some of your travel legs, and once you get west of Austria, you will find fewer and fewer trains. Greece, for example, has almost none. Greece is pretty much ferry + bus land if you don't have a rental car. The rail pass will not help you on flights or (I think) Greek island ferries. It will not cover most buses.

Bottom line: I highly doubt that the rail pass will pay off for you, but you can only be reasonably sure by sketching out a rough itinerary and checking the fares.

I understand the desire for flexibility, and I travel that way myself. However, you can potentially save a lot of money by pinning down the dates of a few of your long travel legs in Spain/France/Netherlands/Austria (and any countries you need to travel through to reach Austria). If you do that and buy those date/time-specific tickets soon after they go on sale, they will be much cheaper than if you buy them just a day or two ahead of time. I'm talking about things like Paris-Amsterdam, Paris-Barcelona, Barcelona-Seville, ??-Vienna.

I avoid the need to make a lot of early bookings by covering less territory on my 4-1/2 month trips. That means I see a lot of (cheaper) smaller towns, and most of my travel legs are on regional trains (cheaper, and with fares that usually are the same, no matter when you buy the ticket) and buses.

How common is it for seats to be full/ you cannot get on the train you want to?

I would be content with spending a couple hundred extra dollars for the ability to wake up and decide to go somewhere, but if I am not able to do that because of reservations, etc., I would not be interested.

Seat reservations are standard for the fast trains between major cities in Spain and France. Also, I assume, for international trains to Amsterdam and Vienna. Less common in Germany (not on you itinerary, but you may cross Germany en route to Austria) and unusual in Switzerland. I don't know about your other countries, but you won't be using so many trains there, and the ones you do use will be so cheap that covering that travel period with a rail pass is not a good plan.

I have definitely seen Spanish trains, in particular, marked as Sold Out on the Renfe website when I was researching the answer to someone's question on this forum. I cannot tell you how often it happens. I think there are still capacity controls on rail pass usage for international trips involving France, so it's possible you couldn't get on the desired train even though it was not actually full.

You can spot-check the reservation situation on the Deutsche Bahn website. Type in an origin and a destination. Choose a date in December and a time in the morning. Do you see an "R" in a circle near the right for a train? That means a reservation is required. Some trips may offer reserved and non-reserved trains, but if they are long trips between major cities, you will usually see a very large difference in travel time. Only you can decide whether it's worth spending (making these numbers up) 5 or 6 hours on trains rather than 2 to 2-1/2, just to avoid the cost and hassle of a reservation.

To see what things look like for trains departing in the next few days, go to the appropriate train website (check Seat61.com for that) and see whether trains are sold out. It's my impression that trains departing around weekends and holidays are likeliest to run full, but I have no data on that.

Really, there is a reason why very few experienced travelers buy rail passes.

Edited to add: I'm not sure why you think you can't wake up and decide to go somewhere and just buy a train ticket that day. Tickets cannot sell out unless the train requires seat reservations, and in that case you're back to the possibility that all the seat reservations will be gone if you want to use a rail pass on the train.

How common is it for seats to be full/ you cannot get on the train you want to?

Not very common, they just keep putting the price up for the last few tickets. People who have to travel at a few hours notice (for work) will pay higher prices.

I don't know if it is still the case, but some companies (SNCF) used to limit passholder reservations on some trains, as they wanted to keep places free for full-fare tickets purchased at the last minute, which gave them more income.

"Reservations for passholders" is a misnomer, which confuses many. What it really is is: "Reduced ticket price paid by passholders", but they don't call it that because they advertise passes as being an alternative to buying tickets.
Either a train has no reservations, in which case a ticket or a pass will get you on, or it is reservation-only, in which case a ticket for that train, or a pass+reservation is needed.

For non-reservation trains they try to plan enough seats for the expected number of passengers. Sometimes, for example in the rush hour, some people stand.
For reservation trains, everybody needs a train-specific ticket/pass+reservation.
There are also trains with "reservation optional", just to make it more confusing.

Despite the need to reserve some trains, the longer versions of the Eurail Global Pass are really very well priced. A youth pays only about $635 for 2 months consecutive or $780 for 3 months consecutive, and that's without any special deals which pop up a few times per year (like this week you pay only $700 for 3 months). You don't have to take a train every day to get good value from this. If you only took a train every third day, you'd be paying about $25 per day of use. While the cheaper countries further east would be less likely to warrant use of a rail pass on their own, that extra month is really cheap at $145.

But, Laura, we don't know how Jacob is going to divide his time. It's not just that trains east of Austria are cheap; there just plain aren't all that many of them (though more in Hungary than in Croatia, Romania, Greece and Turkey). On most days while he's in those countries, Jacob isn't going to be on a train even if he changes cities often. He may not even see a train for extended periods of time.

One example from Hungary: Tickets to Pecs for this Saturday (about 24 hours from now if you're sitting in Budapest) are currently selling for about US $13-17 one-way. One wouldn't normally make a day-trip to Pecs (close to 3 hours one-way, and a lot to see in the city), so the daily cost of visiting Pecs would be no more than $13-17. If Jacob spends 3 nights there, his daily rail cost will be $9-$11.

To the extent that Jacob spends a lot of time in big cities, he'll just be carrying that rail pass around in his pocket with the days ticking off. If he takes day-trips, they are likely to be by bus (not covered) or regional train (often very inexpensive even in France).

Me and my friend have been discussing the pass and feel like having it will open up more opportunity to visit places that we otherwise would have deemed too expensive ie. Day trips with little to no reservation fee...or stop offs at other larger cities such as Brussels, Nice, or Bern that would be too expensive to simply buy tickets too. Am I wrong in thinking this?

If so, would the 15 days in 2 months possibly be a better option, or definitely just buy as we go?

We are definitely not interested in planning much ahead, most likely we will plan the direction of our trip. Such as starting in Spain, moving up through France, Netherlands and then moving east. But we are most likely not going to purchase any tickets in advance.

I think Jacob has the right idea when he writes about wanting to "open up more opportunity." The flexibility and freedom of a pass is worth something, after all, and perhaps more to Jacob & friend than to some of us.

If at the end of the summer they ended up spending $100 more for this versus pre-planning everything and buying tickets in advance, is that so terrible? I'd call it money well spent.

It's a trade off in regards to getting a Pass. I find it excellent plus very convenient when using the night train option.

If you intend on taking night trains, as I do on every trip since 2009, get the Global Pass. It is a good deal for seniors 2nd class. The Youth Global Pass 2nd class is also a good deal,say if you were going Cologne to Warsaw by night train...very doable.

The best day-trips, kind of by definition, do not involve long train rides. Shorter day-trips will often be by bus (usually no pass coverage) or will be inexpensive by train. That sort of trip is not going to make a rail pass pay off from the financial standpoint.

If you're on a fast/expensive train between large cities in Spain or France (not sure about Netherlands, Austria and points east), you probably need a reservation if you don't have a point-to-point ticket. You can get off that train at any stop. But if you want to hop back on another reservation-required train, you can't do that without a new reservation. And If you take a fast train between two major cities, there will be precious few places where you can hop off, because that sort of train doesn't make many stops.

I get the feeling you're looking at the cost of the rail pass, not the cost of the rail pass plus a reasonable minimum cost of the seat-reservation fees you're going to have to pay. And I don't think you're taking into consideration the large number of cash-out-of-pocket bus trips you're going to need during the last half of your trip.

As already mentioned, if you do not make reservations until the day of travel, you may have difficulty traveling on your chosen dates between Amsterdam and France and between France and Spain. Whether there are still capacity controls on single-country trips I do not know.

I used a student rail pass back in 1972; I understand the siren song of "just hop on". But the economics have changed in several ways. It is a rare traveler who benefits from a Global Rail Pass these days.

Jacob, you've received lots of good advice here, including reading up on passes at Seat 61, making a sample itinerary for the purpose of pricing your intra-European travel with and without a pass, and considering the marginal cost of/savings from choosing one pass over another.

I want to emphasize this last tip, which was from Laura. If it turns out that a decent number of your trips can be covered with a pass, going from a select (x days within 1 or 2 months) to a consecutive pass (such as 1 month), or to a longer consecutive pass (such as 2 or 3 months) provides a big increase in flexibility for a small increase in cost per day.

Yes, you do need reservations for many cross-border trains and for fast trains within many countries, passholder reservations can be expensive for high-profile services like Thalys, and some branded or private trains don't accept passes at all. Nevertheless, you will find, especially in western European countries, many trains for which reservations are optional, or not even possible. In pass-unfriendly France, a basic domestic reservation costs €10. In Germany, it's less than €5 (second-class).

Gaps in pass coverage for high-end trains have been widening for decades, but a more recent development is gaps in pass coverage for local trains. Regionalization and contracting-out mean that local trains on some lines are no longer operated by the national rail company. It's really important to confirm that the local operator either respects the tariff of the national rail company, or directly honors rail passes. Some short trips require a local transit ticket (whose price doesn't fluctuate like the price of a long-distance train ticket), and cannot be covered with a pass.

I encourage you to download Rail Europe's "Rail Planner" app and check the "Pass network only" box to find trains covered by the pass. The app also indicates where reservations are required. (Sadly, it doesn't reveal where reservations are possible but not required, let alone where they are recommended. The apps of the national rail companies do reveal reservation-optional trains and may reveal busy days and times when it's wise to reserve.)

Accessing Rail Europe via any of the "Buy" links on Rick's rail pages lets you price passholder reservations. Go to the shopping cart, start a booking, check "I have a rail pass", and indicate that you have a 2nd-class and "global" pass.