I've nearly finished this now, and I have to admit, it's been pretty damn enjoyable. Certainly not perfect, but you can actually tell that there's been some real passion put into the production of the game. The secret item placements are a good inicator of the thought put into it (they're actually pretty tough to spot!), and there's a ton of extras you can unlock (such as 50 Cent music videos, cheats etc).
For an all out balls-to-the-wall blaster, it's defanitely worth picking up on the cheap. Co-op is pretty decent with a semi-skilled player also.