Boring in the start, then lvl 11 is fun, then boring after lvl 13, then it got pretty got from lvl 25 and out :D
The music was kinda cool the first time u heard it, but then it just got annoying. So u should change the speed of the rhythm every 5 lvl or so, that would also stress out the players a little :D
And the color of background should change to "negative" after lvl 25, cuz it gets minor'ly challenging at that point :D
4/5
So long...