I have ridden in a bitless (dressage) - they are absolutely wonderful for teaching a rider not to rely on their hands to get the job done.
What I don't like though is the response from the horse. It is very difficult to use the same soft, finger light aids and get the same response as you would from a bit. Plus they are not legal in the show ring. So as much as they teach the rider not to rely on their hands - they also make the rider amplify her aids.
I do realize that they have their place, and I might school in one maybe 3-4 times a year for S&Gs but for dressage they are pretty useless.