honestly... don't know if they're worth it now, seeing as the new series 8 GeForce cards have a built-in physics processing chip. all they really do is enable more debris and stuff to be rendered in games. doesn't really increase FPS at all due to the fact that it adds more crap while taking physics processing load off the CPU. Right now it's basically a novelty item until something that fully supports it (IF something comes out to fully support it) is released.
i used to have the BFG one... but had to return it due to it overheating my video cards.