AGEIA® PhysX® physics accelerator

Is the AGEIA® PhysX® physics accelerator even worth it? Does it actually make a difference?

No.
Yes....makes most games run worse!

Well your adding extra physics to the pot now, so naturally things can slow down a bit. Ageia had a great idea and it needs to be matured. The idea in fact is now being taken into consideration by ATI and Nvidia with thier crossfire and sli cards. So, Ageia must be doing something right. If you are considering buying the card wait a few months and see what happens. As more and more games start to support it the PhysX processor will start to mature as will its drivers and maybe future versions of it as well.
Just my 2 cents.

If you're asking the question, then the answer is no: it's not worth it.

It's only useful for those willing to spend the money to be "early adopters." The PhysX is nothing more than an expensive hardware BETA at the moment.

You won't see any improvement unless you know specifically that the card will improve performance of a game you play (Apparantly CoV, but I don't know of any more personally).

Then you have to weigh the performance gain against the cost. For most of us, it's still entirely too much.