It's worked great as a PhysX card. It's even only on a 4x PCIe 1.1 slot (Asus P7P55D Pro mobo has two 8x/8x lanes and one 4x lane through the southbridge).
To be honest I've only noticed a framerate drop once due to physx, and that was when there was this huge explosion in Mafia 2 - but all the debris and cloth and smoke throughout the rest of the game it was fine. Batman, Mirror's Edge, Metro 2033 all definitely had no problems. Basically the card is generally only being used 10-30%, and only in really intense areas gets up to maybe 50-70%, almost never does it get fully utilized. Although in Fluidmark, it can only max out at around 83% usage due to the bandwidth issue of the 4x PCIe 1.1 lane.
dip - can't say it affected framerates in the least, but I moved a few games onto it and it definitely helped a lot with load times.