On arrival into Paris at Charles de Gaulle (CDG) we decided we'd get the airport train into town, which we did and it was fine, but it was slow (it took an hour) and it was filthy (really grotty). It was also cheap - I think the two of us got into the centre of town, and then out to our hotel on the Metro for €18.50.
We noticed the AirFrance Les Cars stop outside our hotel and asked the Concierge about it. He said the bus comes every 30 minutes and the trip takes between 45-60 minutes. It cost €16.50/person and we decided that we'd go back to the airport that way and see how it was.
Honestly, it was great. It took us only 30 minutes, which might have been quick as it was a Sunday. There were no other pick-ups after our stop (Porte Maillot) and the bus was clean and quiet. You can buy your ticket online, from the Concierge at the Le Meridien Etoile (opposite the stop), or on board the bus. We'd definitely do it that way again.
Oh, and you don't have to be an Air France passenger - we were flying Swiss.