OCC has published a review on Darkest of Days Performance Testing

What you can see based on the results in these graphs is that when the Physx effects are set to the lowest level, the performance from ATI's cards is almost identical to the competing Nvidia cards. However, the GTX 285 finally pulls away from the HD4890 at the top end. The point of showing these numbers is to show that yes, the game is playable with non Physx-enabled cards. When playing through the game without the Physx effects after playing with them enabled, it leaves a somewhat less than satisfied taste in your mouth. In this series of testing, you can see that a Crossfire profile is not ready for this game, as the dual-GPUs perform at a level less than the single-GPU HD4890. On the other hand, the GTX 295 just flies through this game, showing that Nvidia has the SLI profile in place and working.

Darkest of Days Performance Testing Review