Good, but not great. Short. Lots of untapped potential.
The story is interesting and ramps pretty well at the beginning with a lot of unknowns. The story feels fragmented after a while and it definitely makes me think, based on the game's length, that huge sections of play and explanations (story flow) were left out and made it feel more like a mishmash than a connected story.
I love the weapons and the early (earth) levels. The "powerup" methodology - choosing where to augment your weapons, etc. works well and adds a layer to the game. The sniper rifle was unusually satisfying.
Concerning level design, I have only one major quibble - it seems that the designers put invisible walls across anything they didn't want to bother with completing - making most segments terribly linear. Also, any fall from a height usually results in instant death. Funny, since I can be hit by 4 bullets and lose only 1 bar of health, but fall more than 6 feet and you die......
Still it was fun, and an unusually ambiguous ending makes it even more unusual. So many loose ends and little true resolution makes this a semi frustrating experience in terms of story. The ending battle(s) are very satisfying though.
Another weird thing is the lack of explanation concerning scenarios - you complete a section, talk to the big bad guy, but never see him again, rather battling everyone all of a sudden, with no explanation as to WHY everyone has turned against you, etc. etc.. Still fun for a couple of hours....