It runs like a greased up bronco for me on 5870 Crossfire, DX11 Very High settings.
It sucked on my old 2xGTX 260 SLI. One 5870 ran the game much better, setting for setting, than they did which was a massive surprise given all the Nvidia backing/branding the game has. That 260 SLI destroyed every game I threw at it for a year until it just hit the wall with this.
Anyway, make sure Advanced PhysX is turned off so it's the CPU handling physics and not your GPUs. It might default to the Advanced setting with Nvidia cards, can't recall.