This game is just, oh my god.
If you've never played it, buy it right now on steam while its still half off. This is seriously one of the most fun games I've ever played. The graphics style is awesome and the physics are just insane, if I said much about boss battles it'd be a big spoiler, they're just that good. If you like sidescrolling shooters at all, then this is a must buy. Only issue is the game is fairly short, but its all quality.
To anyone who's played it, what did you think?

http://www.youtube.com/watch?v=BJZAzNKZBew
trailer here ^