Clicker training, even on its most basic level, is so rewarding.
What really amazed me was how quickly you could see the horse's thought patterns working, and how delighted they were to communicate this way.
I think with ANY horse it would work, as long as you follow the "rules" and introduce it properly.
It is so much fun. The links that Liz R. (Hosscrazy) posted are great and I do recommend Ms. Kurland's book.
I think where it made the biggest difference was with my gelding I have now sold. In the ring, in halter, he would just "mug" the heck out of me (touching me, pushing me, beginning to nip) for treats which he KNEW I had (even when I didn't). Once we started clicker training, he knew better, and he knew that after I clicked, he'd get one, but it meant posing! His son works on the same principle and I know he got more than a few compliments on his pose from his last season of showing (he stood still in a very LONG class and posed very squarely, yet still showing his best...it was hot, I was GLAD of this). I have not worked as extensively with the son as with the sire, but when I went to work on Liberty w/the sire, he just ate it up, literally, and whenever my horses were "out" without permission, I could always depend on being able to catch GTO by giving him his cue (making a good example for the others to come in, as well, even though they'd get all jumpy and want to play tag for hours).
Can't say enough about it except read the basic principles and make sure you understand the concept before attempting it. It is not hard, but it is the simplicity which makes it work so successfully.
Enjoy,
Liz M.