I've done taxi from the hotel to CDG, but I also have done RER to Auber/Opera and the CDG airport bus to Opera.

Last time, I took the bus at morning rush hour from Opera to CDG and the bus went down an amazing collection of back roads which enabled it to miss virtually all the traffic leaving the city. I cannot imagine that a taxi driver -- or even a private car driver -- would have been anywhere near as clever. These bus folks have really figured it out!

RER requires one transfer but is also pretty good as there are express trains during the day. The problem is that if you are leaving from terminal 1, there is a horrible shuttle bus connection at CDG. I am told a light rail is under construction.