I don't mind popping a horse on the hind once if he is doing something absolutely unacceptable. It only takes once though, and it only stings for a bit - I've had someone hit me with a horse whip on my "hind," as hard as I would pop the horse for discipline just to see.
I use one when lunging, but only as an aid. I never touch them with it. I'll also use it move the horses from one paddock to another, as an aid. It makes me seems much bigger than I am, and they listen a bit better in a group.
You shouldn't beat your horse, but as an aid or a disciplinary device, I think it's ok - so long as your horse isn't -injured- in the process.