For me, GPU Physx is one of those things that isn't incredible, but is cool enough for me to want a card that supports it.
I absolutely hate the fact that nvidia has possibly (most likely...) limited CPU physx to single-threaded usage to make GPU physx look more appealing, but it doesn't really change the fact that some games make good use of it, and I have a weak spot for getting the most features possible out of good games.
The list of actual GPU-Physx accelerated games is incredibly short:
But Cryostasis, Batman and Mirrors Edge alone make it worth it to me. I really like what I've seen of the effects in these games, and the games are good enough for it to matter.
I'd prefer there was an open standard that didn't use any lame tactics to manipulate the market, but for now, there aren't any alternatives and I'm going to keep buying the products that have the features that appeal to me.