I picked it up over the weekend and have put a few hours into the single player. Maybe I just haven't kept up with the massive scale FPS like CoD lately, but I think this is pretty enjoyable. I'm not dying to get back to playing at any given time, but it has a certain level of intensity that I don't think many games capture at this scale.
Actually, in a lot of ways I feel like this is the game that Halo 2 should have been. While that may sound odd, there are a lot of things here that strike me as similar to
this old ten minute trailer of Halo 2 from some E3 that ended up being way cooler than the final game. Battles almost move and flow exactly like the stuff in the trailer, including the banter from other troops, moving from bigger skirmishes to close quarters stuff on the side streets, and larger "background" objectives like taking out distant artillery/AA positions. Of course, the visuals are different (and a lot better), but the setup of an invasion of a large sci-fi city remains the same (even if the roles are reversed). The funny thing is that Halo 2 ended up being a step down from the trailer and Killzone 2 isn't quite up to the level shown in it's first trailers. Maybe we'll finally get that game in four years or so. It seems we don't get things promised until a generation later anymore.
But yeah, I've enjoyed the game a lot so far. I've always wanted a FPS that centered around some sort of planetary invasion and this is feeling the bill nicely.