CDG to Gare Montparnasse -- Metro or LesCarsAirFrance?

Scott,
We are doing the exact same thing. But instead, you can catch the TGV right at Charles de Gualle airport rather than trying to find your way down to the Montparnasse station. But your timing might be off to catch the TGV since your arrival is 9:40... so never mind. We're getting in at 6:45 and catching the CDG train to Tours without having to get off. So, there are lots of opinions. Let your luggage + back be your guide here. RER/Metro means lugging your luggage and 2 trains with one connection... and no view. LesCars means sitting and watching Paris go by and not lugging. Look at the price difference and say to yourself... if I could pay someone to lug my luggage across Paris for me, would it be worth __ euros?

Very helpful, thanks. Related question: Later in the trip my wife and I are staying in an apartment about a 15-20 minute walk from Gare Montparnasse. We need to be at CDG airport by about 8:30 a.m. on a Tuesday morning for our return flight. Would it be better to take the Metro and RER (no worries about traffic delays, but stairs and a stop), or walk to Gare Montparnasse and take LesCarsAirFrance?

Jean-Paul has it just right. With RER and Metro you have to make at least one change, schlepping your bags through a station with probably a bunch of stairs, and then as I recall it's a pretty long moving-walkway ride underground from Montparnasse Metro to the train station itself. And of course all you see is trains and tunnels and busy people. On the bus your bags are taken care of, no changes, and yes there's plenty to see from the bus, specifically much of Paris. And the bus lets you off close to the station entrance, within a block or so I think and no street crossing. For me there would be no question, especially if I were traveling alone and paying only one bus fare. Especially in the morning after an overnight flight.

If arriving at CDG at 08:30 is a full three hours before your flight home, then taking the Air France bus will work just fine. The first bus leaves Gare Montparnasse at 06:00. However, if I were in your position, I would take the Metro and RER. Depending upon the exact location of your apartment, you could take the Metro to Denfert-Rochereau and transfer to the RER B there. Or you could take the Metro to Gare du Nord and transfer to the RER B there.

I support Les Cars Air France 100%. I took it this last June and I've taken it several times before that. You can even purchase your ticket in advance online and save a few euros. Once you get your luggage it is marked everywhere where to find the bus stop. Note, it can be quite a jaunt to get there, but it is totally worth it.

For your return to the airport from the apartment, I agree with Tim that the RER from Denfert-Rochereau would be simpler than walking 15-20 minutes to the LesCars bus at Montparnasse if the apartment is near that (or another) RER-B station. (Be sure you get on the RER train marked for the airport, there's another B train that goes toward it but then elsewhere). But if the apartment is west or north of the gare Montparnasse then I'd stick with LesCars. Does that make sense?