This was quite fun! I played the web version, and the notes were tolerably in sync with the music, which I think is impressive for the web. I thought the beats were pretty fun too and had me bobbing my head along. I wish there were more levels, or maybe some leaderboards to add some challenge.
I think the UI was pretty clear and well put together. I wish there was a little more going on with the alien band - granted, I wasn't looking at them too much, but still would have been cool to have little animations in time with the beat to show them playing along.
Your cover art doesn't do the game justice, but I was surprised how much I liked the game when I booted it up :)