CDG to Gare Montparnasse

In my opinion,it's way too close to make it. All kinds of things can go wrong on your flight. Not to mention potential traffic problems from CDG to the station. Also, you just don't walk right into the station and hop on the train. It's a huge station (as are all the Paris ones) so add that on top of the jet lag and the crowds,to me,it's not enough time.

RER plus Metro would probably be faster than ground transportation ( taxi). You can speed up exiting the airport if you don't check your bags. Slim down your personal items to fit into two carry-on size bags and you can save up to half an hour (or more). Follow sign to RER.

The blog www.parisbytrain.com can offer some advice plus it has photo guides to big stations such as Montparnasse.
Is it possible you could take a train from the SNCF station at Charles de Gaulle?
http://en.voyages-sncf.com/en/

Where are you heading to?

This is really a game-theory question and an important piece of information is how cheap are the tickets when bought 90 days in advance vs the walk-up price.
For instance, I see that 2 tickets to Tours (St Pierre-des-Corps actually) for early afternoon Tuesday Feb 7 are 30 EUR. The walk up price is 114 EUR. With that kind of spread, it might be worth it to try. The worst that can happen is that is you wasted 30 EUR, which is about half the cost of a taxi from CDG to Gare Montparnasse.

And we don't really know what the train schedules will be in August, since there will be new schedules in effect after June 11, and we don't know them yet. Your flight times could change too. Airlines are doing that all the time. So it is a bit early to be worrying too much about it now. Wait until you are 3 months out and you are reasonably certain of the flight times and the rail tickets are available to purchase, and you will know the prices.

Thanks to each of you.
Sam, yes of course I'll make the final determination based on information closer to our trip. I'm not worrying, just weighing possibilities.
Southam, unfortunately, no, we can't get a train directly from CDG that would serve this exact purpose.
Swan, if we do this I'll be researching Gare Montparnasse as thoroughly as possible ahead of time to cut down on "what now?" time. RER/metro is something I should at least consider. We're experienced in that regard but I'm not crazy about making the trip with luggage while just off the plane.
Carol, all good points. I know it's iffy.

Since your trip is in August you have time to mull over possibilities. So I'll tell you what we like to do.

If the destination is not that far away then a walk-up train ticket may not be much more expensive than advance purchase, and there are often frequent trains. So we buy at the station.

When an advance purchase ticket makes sense then we spend the nite in the arrival city. Breaking the trip this way takes a lot of the stress out of the trip. Planes may not arrive on time, but they usually arrive the day they are supposed to. All we need to do once we arrive is get to our hotel, and after that we can walk around to get over the jetlag a bit. The next morning we take our train out of town, no worries about missing it.

Thanks, Bill G. We've done that before (spend the first night in the arrival city) and it works well. This is just an idea for a different start. We thought we'd get on the train, then go to the hotel and do the walkaround in a different city.

Nothing's written in stone, just an idea for a different start. If it seems like too much of a stretch to make this particular train, we may just pick a different city with a more favorable train schedule.