It was OK.
The idea is great, really - and the programming and art were quite solid, so props for that. But it had a couple of flaws, too.
First of all, lag. There was quite a bit of that; I initially thought that this was just due to Flash being Flash, but there's quite a few fast-paced action games on NG I can play without problems, so it can't be just that.
The lag was made less problematic by the fact that the arrows were pretty forgiving, at least; unlike in games like DDR, you could make them count even if you didn't hit them all that well. But lag nonwithstanding, I think that this is something you could improve: make the timing windows smaller, and at the same time, introduce different timings instead of just "got it" or "missed it". This is a rhythm game, after all, and it's all about getting a good rhythm going.
Of course, getting a good rhythm going also requires high-quality patterns, and I felt that the ones I've played so far (the first six songs) were all subpar. They seemed random, not connected to the songs, and without any sort of "theme" to them; often, I even had the feeling they weren't synced up with the music. In short, lots of room for improvement there.
While you're at it, be more careful with the "press the arrow keys this or that way" sections, too. Don't randomly switch back and forth between those sections and the regular mode all the time. And PLEASE, explain how you're actually supposed to hit the keys in the "lying figure 8" sections; I still haven't figured that out.
Work on all that, and you'll have a winner. But I'll say it again: games like these are about rhythm. A good, precise, lag-free engine is the foundation you need, and high-quality patterns are what makes or breaks the entire game. The potential is there in this one, but it's not fully realized yet.