CDG to Gare Montparnasse

Le Bus Direct Line 4.
http://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/public-transport/le-bus-direct-line-4

I second Sam's post. It's fool proof. Takes a bit of time, but it's so much better than lugging your luggage around on the RER.

Same recommendation. After a long flight,it's a painless way to go.

We took the bus in April on that route, and made it in 2 hours during the Parisian strikes. I would recommend the bus, but wouldn't count on 2 hours. For one thing, we were helped by the fact that we had already gone through immigration in Iceland.

How long depends on the day of the week. For example Sunday will be twice as fast as Tuesday.