City Passes! Good investment?

It really depends on what your interests and plans are! My recent experience:
In November, no pass in Venice. It is so compact, and so many of the sights are free churches, I'm not sure it makes sense.
In Feb I did the Roma pass and it was totally worth it. In 'high season' it would only get more 'worth it'.
In April, I skipped the Paris pass and that was worth it. Shorter trip, interests outside what the pass covered, plus transit is cheap on a strip card.

You are right that they vary a lot. You have to look at what the passes cover, what you are interested in, and how much advantage they give you, not only for price but also for queue jumping (time and the sore feet tax paid when standing in a queue for an hour). And the dates of your trip--if it's high season, it might be worth 10 Euros to not stand in line at a single site (e.g. San Marco). You need to think about how many museums you can actually do in 2 or 3 days. Most people draft up itineraries that are way too ambitious and overloaded. You need time to just meander and hang out to enjoy Europe. If a pass might help you do that by giving you flexibility it's worth it. Or it might not because you feel like you need to 'get your money's worth' and you over-plan. I do think that the RS guidebooks do a good job of helping figure this out--whether it's worth it or not for a particular city.

That said, I do think in most European cities, a transit pass can really be worth it, at least for the portion of your time in a city where you know you will be on the move. You pay once and you can hop on to whatever makes sense, as you please. The flexibility is worth something.

Most you can get on the spot, too, so if you change your mind along the way you can change your plan. So try one on for size and see how you like it.

Have a fantastic trip--you have some wonderful places on your list!

Paris Pass is a definite rip-off. Ok, suboptimal choice. Very overpriced for what you get. Individual parts are cheaper to buy on your own.

At the same time, we made good use of the Paris MUSEUM Pass: that one doesn't include public transport so is much less expensive than the Paris Pass… which I agree is ridiculously overpriced. But if you're not interested in what the pass covers, then it's not a good purchase for you. We bought and shared a carnet of 10 individual tickets for the metro (they work on buses too).

You shouldn't need any transport in central Florence as it's not that large, and we bought individual tickets for the very few times we've felt the need for public transport in Rome but we've only stayed in the center (and decided against Roma Passes). Where is your hotel?

One note if going during high season: while a pass will allow you to skip ticket lines, it will not allow skipping the security-check queues at attractions which have them, and some of those can be long (i.e. the Colosseum in Rome).

Also for Rome, reservations must be made for the Borghese whether using a Roma Pass or not, it doesn't cover anything at the Vatican, and most passes I'm aware of don't cover transport to/from airports.

The offers and the value comparisons are very different in every city, as well as dependent on your plans. I'd guess that separate transport passes and museum passes are often cheaper than a complete package, but you really have to confirm as you go. Rick's guidebooks always include a summary of pass options that will make it pretty clear whether we consider them a slam-dunk, a maybe, or a no-go.

Thank you Laura, I have already purchased "Rick Steves Best of Europe 2015". but I haven't delved to deep yet.

For me a key factor is that if I go to a museum, I tend to be there a long time, reading all the high-level explanation on the wall (if in English) or listening to the full audio tour. The larger cities with the big-buck sights usually offer passes for 1, 2 or 3 days, and you have to get to a lot of sights in that condensed period to make them pay off. In theory, I'd like the ability to stop in to a sight I'm not sure about for "free", but in reality I'd never have the time, because I'd still be wandering around the Louvre, the Prado, etc. I think passes work much better for tourists with long hit lists who mostly want to see just the highlights in the museums they visit. For them, the passes can be real bargains.

I'm especially leery of buying a pass that covers only part of my time in a city, because I feel that it constrains my plan of attack. Maybe the weather will be glorious on Day 2 of the pass and I'd rather do outdoor activities that day. Too bad, gotta use the pass. The line for the Pergamon in Berlin is over an hour long? What a shame; today's the last day the pass is valid; queue up.

There's also the issue that passes may not cover special exhibitions in participating museums. I understand the logic: If the special exhibition costs extra, why should the pass-holding tourist get that for free? But in Berlin I was required to pay the full regular+special admission charge at the Alte Nationalgalerie, not just the special fee. That was pretty infuriating, considering that I had gone to the museum that day specifically to take advantage of having the pass. If I'd known about the policy before standing in line for so long, I'd have gone somewhere else that day and not wasted hours of my pass-validity period.

There are a few small cities that have non-timed passes, or such long validity periods that for the typical tourist, they effectively don't expire (example: Orvieto). Those work well for me if I'm going to be hanging around awhile.

Some of your cites are small enough to skip public transport. For example, Venice, Florence, Nice and Barcelona can be easily toured on foot. You can probably get by without a pass in Madrid or Rome, we didn't use public transport that much. Paris and London are cities were you will likely use the Metro and Underground. Not sure what the passes cost. I suggest a Rick Steves guidebook.

Also, watch out for pickpockets they are terrible in places like Rome, Paris, Barcelona and Madrid.

For transportation, look at these:
London - oyster card. it's convenient and saves money. You'll want to use the tube and/or buses in the city from place to place. It's too spread out for walking.
Paris - you have two options, buying tickets in packs of 10 (carnet), or Navigo passes. The Navigo is convenient IF your travel dates work out. The pass is for 7 days from Monday through Sunday. If you have 4 days with the pass, you will at probably break even. Sights in Paris are spread out so you'll want to use the metro and buses a lot. If you have only 3 full days with the pass, it may or may not be worth it for the convenience, especially since you then just hop on a bus for a couple of stops and give your feet a rest without thinking about the cost.
Venice - vaporetto passes. Unlike other passes, they are based on hours (24, 48, 72) from the first use. Individual rides are expensive and the vaporetto is fun to ride up and down the Grand Canal just for the views. Take it across the lagoon (at least for the ride) to Burano and back.
Barcelona - some of the sights are spread out and taking the metro will save you a lot of time. You can get T-10 card (good for 10 rides - not sure if it can be used by 2 people at once).
Madrid if you are staying centrally, you may not need more than a couple metro/bus trips

Museums.

It depends on how many museums you want to visit. You are moving around a lot. You need to go back to your hotel in the afternoon to get your bags, then go to the train station with enough time for delays and to find your train. Airports take longer to get to and to get through. Even with a 6 or 7 pm departure, you may not have a "full day" especially since you may be moving during afternoon/evening rush hour. For instance, if you are near St. Mark's Square in Venice, the vaporetto takes an hour or more to the train station. Walking is 35-40 minutes (without luggage) up and over bridges and only if you don't get lost.

I count over a month of nights, you're going to need down time - don't underestimate how tiring it is to keep packing and unpacking, getting oriented to each new city: language, signage (Europeans excel at hiding street names), local transportation (how to pay for it, how to use it, what the stations look like), just finding your way from sight to sight to hotel without making too many wrong turns. You're going to have to shop for supplies, with labels in foreign languages, sales people who don't speak English, and quirky things. I spent ages looking for nail polish remover in Vienna in every supermarket and pharmacy. I finally ran into a couple of Australian women who looked like residents and they explained that it's only sold in specialty stores for cosmetics and toiletries.

So be realistic about how many museums you are going to visit. You may well want to spend more time doing other things - walking tours, markets, etc., and less time at "sights." And the further into the trip you are, the more likely you are to be a little travel-weary and/or museumed-out. Sometimes you can skip the line at a sight with the pass, but not always. Sometimes you have to make a reservation even with the pass.

One more thing . . . :-)

I haven't read the RS Best of Europe. Maybe someone else can tell you how much practical information is included.

I think it's essential for the first-time traveler to read RS Europe through the Back Door before you can't change your plans.

You might want to invest in Rick Steves' Italy, Paris and Spain guides.Much more info than the "Best of Europe" book, I believe.

As to passes, top marks to Chani on the advice, especially transportation in London, Paris, and Venice.

Paris MUSEUM Pass was a great value for us. You have to do the math and decide if you will use the full value so build a little chart and figure it out.

Roma Pass is good if you go to the two highest priced sites first: Borghese Gallery and Colosseo/Palatino/Foro Romano. If you are skipping one of those, it probably does not make sense.

You can buy transit tickets in Rome that are 24, 48 & 72 hours. Very good idea if you are 6 miles from the Vatican. You will use public trans every day. But IF you get the Roma Pass, 72 hours of transit are included.