Le Bus Direct from CDG to Gare Montparnasse... your input please.

If you are taking a taxi to Gare de Lyon (the Right Bank), the set rate is 50€ for up to four persons and their luggage with a 4€ per person charge for the 5th or 6th passenger. The charge to the Left Bank is set at 55€. I don't know how a taxi would get to be over 100€ unless you more than 8 passengers and even then two taxis would probably be cheaper unless you have a number of infants in your group.
Of course, you have to "find the rest of the way yourself." You get dropped off at a bus stop and unless your accommodations are across the street, you will have to figure out how to get there from the bus stop. Sometimes the bus arrives on time and sometimes it is late -- that is a function of surface traffic. I have not used it in a couple of years but I personally preferred the bus to the RER B if I was going from CDG to someplace near one of its stops because I did not have to make a transfer and did not have to worry about my luggage or pickpockets. If getting to Paris quickly is an important factor in your transport decision, then any route subject to surface traffic can have delays and the RER is usually going to be the better option.

Yes, anyone who was expecting the bus to drop them at their hotel entrance was misinformed. Any road transport will have some variability in times depending on the level of traffic.

It is a good way to reach Montparnasse, as any RER/Metro route is likely to include at least one change that would be awkward with heavy luggage.

It's fine, very comfortable, and will get you to the station to get your train, but via a long route going to the Gare de Lyon first. And it's 18euros each, so 36 for the two of you. The taxi is a fixed rate of 55 euros because Gare de Montparnasse is in the left bank, not 100. Be sure you tell the taxi driver that you know the price.

It's comfortable, convenient, less expensive than a taxi and less hassle than metro/RER. You sit up high so it's even a bit of a sightseeing trip (although of course the routes aren't designed for that). We took it from the airport (CDG) to Gare de Lyon in 2016 and it couldn't have dropped us off any closer to the trains without actually going inside.

Less expensive than a taxi if you're two people or solo . . . . (it's 18 euros one way per passenger on the Bus Direct).

More expensive than a taxi if you're three people or more.

A taxi to the Gare de Montparnasse area from CDG is fixed at 55 euros for four people; four euros for each additional passenger. I don't know who's telling you around 100 euros, but they're ripping you off.

We've taken Le Bus Direct to Montparnasse a couple of times, and I had no complaints. And we bought tickets as we boarded the bus. In each case we were taking the train from Paris, so Montparnasse was our final Paris destination. Other times we've taken the Metro.

Kim is right of course. Le Bus Direct is cheaper than the taxi only for one or two people, about the same for 3 and more expensive for any larger group.