Very cool!
Now, I like the style, and the music fits well (XD)!
Here's what I think, though: the key system is a bit, well, clunky. How about simply mousing over or clicking on them instead? That would work well, because when I got a fast "A" circl, I was unsure which button to press, as I don't know my way around the keyboard very quickly (I can obviously type fast, though).
Overall, good game, good concept, looks great! Nice job!