I enjoyed the online game play of Killzone. Controls are easy to get use to, don't be turned off by a FPS that's on a handheld system, it handles very well.
I enjoyed the game so much that I decided to buy it and add to my collection.
I didn't play campaign mode. I rented this for online play only, and I must say if your looking for an online FPS for your vita, this is it. And this is coming from a guy who has played many FPS on the ps3 and xbox 360.