I used to hate FPS' since Quake, but I have to admit, I do like the CoD series. A lot of my friends play it, which is nice, but then they play Halo too, and (story aside) I don't really like that.
What I was really hoping for was something like MoH Heroes on the PSP, which I had hours and hours of fun with. I might even go back to that TBH.
But yeah, a decent FPS would have been nice to play on the Vita, though it would be even nicer if any of my friends had one... everyone used to have a PSP, and now it's just me and my Vita and hardly any good games.