Regarding using the RER from CDG into Paris, I agree that it is easy and very cost efficient. But I don't think that anyone mentioned this tip: Have some Euro coins with you so that you can use the ticket machine at the RER station at the airport. There can be long lines at the manned counter, and the machine doesn't take non-EU charge cards, or bills.

Just get a coffee on your way through the airport, and make sure you get the right amount of coins. I believe that the CDG website will tell you the cost of a fare into the city on the RER. Or surely a Ziner knows.

Have a great trip. Debbie in Pittsburgh