Just spent the past 3 hours playing it, and I must say it's pretty good so far. The campaign is actually pretty interesting and the challenges are, appropriately enough, challenging. Also, thank the maker, a PC FPS with lean! my old friend, how I have missed you so.
I haven't actually played any MP yet, but I'm waiting till tomorrow (today...) when people are actually all on. So far, my only real complaints are that the classes really play just about identical to each other, and that it feels like the health boost you get from the "heavy" body is just plain pitiful when you consider how much speed you give up, even compared to the medium body. Other than that, the gameplay itself is tight, the customization is just as deep as it was promised to be, the art style is awesome....really, so far the good outweighs the bad by a mile. Can't wait to wake up tomorrow and play some more, hopefully with some of my steam friends.