RER B from CDG to Denfert-Rochereau, then line 6 to Gare Montparnasse. A bit cheaper (10,30€, the ticket is valid to any metro station), and not dependent on traffic, but you need to change trains, with a couple of stairs to climb.
Note that all RER B trains from CDG go through Paris and stop at all stations inside Paris, including Denfert-Rochereau. However, some trains are non-stop to Gare du Nord, while other have many stops. It's usually worth it to wait a few minutes more for the non-stop (much less crowded).
Note also that there are two metro lines going from Denfert-Rochereau to Montparnasse-Bienvenüe. However, you really want to use line 6 and not line 4: line 6 gets you very very close to the train station, while line 4 gets you quite a distance away, with lots of stairs, corridors and a long moving walkway to get to the train station.
When you exit customs at CDG T2, it's very easy to get to the train station: from terminals 2A, 2C, 2F, take a right after customs. From terminals 2B, 2D, 2E, take a left. Walk straight ahead following the signs to the train station, and you'll get there in a few minutes.
Make sure you use use the windows/machines for the RER (quite clearly marked "trains to Paris") and not the long distance ones.