I heard about this game during Summer Sale 2014 and I am so happy that I did. It has a great, original concept and the soundtrack is wonderful.
EDIT: The game seemed very difficult at first due to its unorthodox gameplay, but I grasped the controls quickly and the first three difficulty levels (there are six in total) were a breeze. I now have to beat the last two, and now I wish I had a guitar controller :)
The challenge mode is also very satisfying. Their difficulites vary greatly as well. There are only 10 tracks in the whole game, but the challenge mode does manage to spice up the game. The challenges are not necessarily "more difficult" than their corresponding campaign levels - sometimes they are easier, sometimes they make you wonder "...is that even possible??".
Also, if you have FPS problems you can just try listening to the music and keep the beats without concentrating too much on the screen instead of meddling with your settings.
Oh, and completing all of the Steam Achievements is very, very difficult (practically impossible for me, but I don't know about you, dear possibly-obsessive-shooter-loving-review-reading-person. There's still hope for you.)