I've taken the RER many, many times between Gare Nord and CDG. Never an issue. There are, if I recall correctly, a few "express" trains that don't stop along the way. I'd recommend those for anyone concerned about their safety. It's certainly more reliable than a taxi (since you don't have to worry about traffic). Of course, it is France, so you do have to consider that plans could change at the last minute due to a strike.