Very often the RER B will get visitors into Pairs much faster than will a taxi which can be slowed due to traffic, sometimes very heavy traffic. Recently (last week actually), friends of ours spent over two hours in a taxi to reach the 5th arrondissement from CDG after an early morning arrival from IAD.
The RER can be crowded at times but I do not agree that the ride is somehow rougher or more jolting than might be a taxi driving through the streets of Paris.