I've ridden with him three times, and learned a lot each time.
He's very patient, and works well with a lot of different horses. I went with my 20+ year old Arab, who didn't have a ton of dressage background (and I'm definitely not high level). He had us doing really nice half passes by the end of our ride, which we'd NEVER done before. I'd asked specifically for help with canter departs, because I was really struggling to keep them balanced, and he had me work through a series of gymnastics that made a huge difference.
I couldn't do 100% of what he taught when I was back on my own and working with my trainer, but I could get enough pieces of it to work that it made a noticeable difference.
He's got a different style of teaching - he'll spend a lot of time explaining the biomechanics, then just working on your position and aids, as well as the horse's balance at the walk until he's confident that you're exactly where he wants you. It was interesting - we probably spent 2/3 of the lesson either standing still or walking (with a lot of intense exercises), and when we moved on to more advanced stuff, it was easy!
Oh, several riders did mention that they had some trouble with his accent (heavy French). I work with a lot of Europeans, and really didn't notice it that much, but FYI.