Bought it expecting some kind of Left 4 Dead clone and turns out to be a pretty fun game actually.
I don't fully get behind the unlocks but I guess you have to have some form of progression system, even though it is a bit too slow for my liking and there just isn't enough missions to play really.
On the other hand it is kind of good that there aren't too many as it can get repetitive really quickly if they overdo it.
Generally I'd say this game if fun for what it is and worth a purchase if it's cheap on a sale.