This is one of the bigger video game surprises of the year for me; the level of polish and depth in this game was far beyond what I expected, and I had no trouble playing this for hours on end without getting bored, which is a rare feat for me.
Essentially, this is like Diablo in FPS form in a post-apocalyptic wasteland with Unreal Engine 3 cell-shaded graphics. The main idea of the game is to level up, gather loot, kill bigger enemies, and basically just grind your way to the top. That sounded a bit dull to me at first, but then I realized that when you slap this type of gameplay in an FPS, it actually ends up feeling fresh and motivating. You can really see the difference when you find new guns or level up. Slap on Coop and a surprisingly decent story, and you've got a winner.
Now, the main disappointment for me was the ending. There's a pretty major and difficult battle to the top of a snowy volcano, and then when you finally get to what the whole game has been building up to, you get a quick and confusing ending that isn't explained at all. I suppose that means that DLC will explain it, but it was still pretty disappointing. Also, as good as this game is, it isn't revolutionary in any sense of the term: it merely mashes together pre-existing genres and mechanics into a well-polished whole.
Still, one of the best Coop experiences of the year, so far.
In a sentence:
Surprising levels of polish, depth and fun, but ends rather oddly and doesn't really push any boundaries.