This game might not have tons of depth, and I guess that later bosses could be more inspired, but..
Oh, it's so much fun. So much fun. The core loop of the game is executed perfectly, and that alone makes the game insanely satisfying and addicting.

And the soundtrack? Maybe it's not Mick Gordon, but it's still brilliant. I don't mind turning game on just to "check settings" purely cause of how great the menu music is - and that goes for all of the game, really, but the menu music (also present in level 5 and in ♥♥♥♥♥♥♥♥ trailer) is definitely my favourite.

I just HAVE to recommend.

Update post Metal Hellsinger release: so, Metal Hellsinger is a fine game, and has really amazing soundtrack, but... BPM is just a better rhythm shooter. Audiovisual feedback is just much greater, and rhythm mechanics actually dictate more than just shooting.