The RER B line from CDG departs roughly every 10 minutes until about 11:50 pm or so on a Monday evening. Think of it as a commuter train.
While it isn't a difficult journey, it is not the same no-stress smooth experience of a taxi or private car.
IMO, taking the RER B only makes sense if you are traveling relatively light, a carry-on and personal item, and staying near one of the many stations along the route. Otherwise a transfer to Metro or bus or a taxi would be needed to get to your hotel.
While this isn't a complete list of the stations. for most visitors to Paris, the stations of interest would be
- Gare du Nord
- Chatelet - Les Halles
- Saint Michel Notre Dame
- Luxembourg
- Port Royal
- Denfert Rochereau
The current cost is €14 per adult each way.