As a guitarist who used to play in a band, I love love LOVE the concept behind this game. I had a bit of trouble discerning the correlation between the mouse gestures I was making and exactly what sounds would get played; there were some consistencies, but I didn't feel like I knew exactly how to play certain riffs quite when I wanted. Still, when things worked out and I was harmonizing well with the music, oh man, it was awesome!
Looking forward to seeing more music games, I really love when a game forces the player to think differently -- in this case, with their ears!