Good, short, difficult.
I can understand why you wanted to make it so hard, because since it was short the harder it is, the more hours people will clock trying to beat it. Good idea, but you started out to hard. Hence all the "too hard" complaints. One song before easy (very easy?), that most people could beat one the 2nd or 3rd time. Took me about 8 tries to beat easy :P, then like 5 tries to beat medium, and then about 8 tries to beat hard. Not that easy was so hard, but because I never got any practice on an easier song, so I had to keep retrying not getting very far, which was frustrating, and is what most people are mad at you for. Just start at an easier point next time.
Maybe it's just me, but I didn't like the diagonal arrows. It just didn't seem natural with the arrow keys, I would rather have it with just 4 directions, and maybe more notes or something to even out the difficulty. Or instead of diagonals just have an enemy coming from the top and side at one time.
The insane mode was ridiculous. No one has beat it, nor will anyone probably. It was also a rather lame song for the amount of work it took to unlock it. It a lame song for any thing really. You should have just made a really cool loop, maybe 10-30 seconds in length, and just increased the speed after each loop, and make the song never ending. When you die, you can send your score in, that would be the best way to do a highscore thing.
I heard the super secret sucked, so I didn't bother to unlock it. More unlockables would motivate me more, like you unlock bonus things for the amount of total "bubbles" you have.
Some 'bugs' seemed offbeat with the music.
That's about it as far as my complains go, good job on this. Keep those on mind if you ever make another rhythm based song.