Can't remember the time of the first RER from centre of Paris to airport, so it depends how "early" your early is. If I recall , the Roissy Bus which leaves from close to Opera starts a little earlier. Last RER used to be around midnight ( but I am talking 1996 so things may have improved) so if you get in a bit late you are stuffed. Road traffic got worse by a factor of ( felt like ) 4 between 92 & 2000, I wouldn't recommend trying to drive in and out of town too often, it is truly awful for many hours each side of the rush hour. What was a lonely drive @ 0530 in 92 became quite a chore 8 years later, and we are 8 years further on now, so I guess it sucks. If you fancy a different version, 20km or so North of CDG, Senlis is a very nice town, albeit a little pricey ( as of course is the centre of Paris) and you won't feel too out of place. It has most of the charm you would expect to find in a quaint English country town and then some. For there you need a car of course, but drive to Airport is relatively painless apart from the Q at the exit @ CDG. the nearest RER is however a few km away, which keeps the less-desirables away a bit at least.