Soooooo Rhythm Shooters. Sounds like a crap idea. HOLY SHIT IT IS AWESOME!!!!!!
It's basically Doom, but shooting in time to the music does extra damage, dashing in time to the music is better, and reloading in time to the music is twice as fast. There's also a powerful finisher that is (yup, you guessed it) in time to the music. It shouldn't be this much fun, but when you pull off a slick dash kill 180 headshot finisher reload kill some more......... it's amaze balls. Download the demo, forget what you think you play an fps like and go ham.
The music is epic too, Serj Tankian etc, depending on the multiplier depends on how much music plays, 16x multiplier is EPIC music time so play with headphones.