DK2 rocks for any game with a cockpit -- space sims, driving sims, and some Mech Warrior like games.
FPS'... no so much. It's real easy to get queasy (for me anyway, YMMV). I'm hoping this becomes less of an issue over time but since there are other experiences out there that really excel with the Rift I'm not too concerned.
Oculus also has to come up with an standardize input system to enable more complex experiences. Something that is also positionally tracked (like the headset it) would be ideal since once you have your headset and headphones on fishing around for your mouse/keyboard or game controller is a pain.