Great gameplay, but a bit short. However, it's super cheap so who cares about that.
Now - on to the actual game. In sort: it's awesome, in awesome: it's short. And that's the biggest shortcoming of this little gem (no pun). You play through it in under 10 hours (think it took me like 6 or 7, which totally makes me awesome quick) after which you can replay all the levels (you just pick em from the menu) for achievement / guns / powerup hunting in single player, or some multiplayer brawling.
It's a really simple and quick playing game - point and shoot, and it works precisely the way it should. (I really hate the following words, mainly due to the fact that people use it for Italian cars which then start to rust and break down, but here goes:) It feels right, it has spirit. Simply put, it's a blast to play. Lots of enemies, which can easily be evaded once you get the hang of it, nice little puzzles which can be tricky, but never overly complicated. It's just really good simple gameplay. Definately worth buying/playing, excellent value for money.
So my advise, wait for the next steam sale for this game (which I'm sure will come) and purchase this little gem.
On a sidenote: I purchased this game on sale expecting some oldscool 3d tombraider action, gotta say I was VERY pleasantly surprised.