I caught the tail end of a clinic at last years Equifest. It was very interesting, from what I saw, everyone could benefit from it. Not just the English arena style riding, but western and trail riders as well.
Didn't have an instructor, but got the book and practiced on and off a horse. I swear by it and have benefitted from it 100%. Lent my book to someone (can't remember who) so now I'll have to buy another just to refresh my old memory!
I did a clinic a few years ago. We did a few of the exercises in the 2nd Centered Riding book. It got me thinking differently about how to properly use my seat. If it's not too pricey, do it. It's fun.