For the Air France bus you have:

Positives

• You can buy the ticket on the bus
• Once you're on it, you go in comfort to your destination without having to schlep luggage in the RER/metro (if you're checking bags that means you'll have plenty to carry, and you're not going to have fun taking them through the metro, up and down the stairs, and in the crowded train cars)

Negatives

• It runs every 30 minutes and takes about an hour from CDG to Montparnasse (that will vary depending on traffic conditions). So, in a pseudo "worst case" scenario, assuming you arrive just after a bus has left, you would have to wait 30 minutes for the next one (assuming that it's not late)... for a total of at least 1-1/2 hours waiting and riding.

So much depends on variables you can't control: Whether your flight arrives on time, how long it takes to get through immigration, baggage claim, and customs, time spent getting to the bus stop or train stop, transit time... If all goes well you can make it, but be sure to have a back-up plan in case you miss your train.