Killzone is definiately the best FPS on the Vita, although thats not saying much, I've only really had time to put an hour into the campaign but I have to say the devs have done a good job so far. I'll get to try the multiplayer too this weekend and although thats only 4v4 on the maps from what I hear thats enough.
Controls are similar to most FPS's, although it might just be me but the ability to crouch behind objects sometimes seems to take a bit longer than I'd expect, however thats a small niggle. Oh, and if you do buy it, put 30 mins aside for the update patch to download and install before you think about playing!