Could be much better.
~ Animation/Graphics ~
The Good Points - I thought the hit detection was rather good. There wasn't any time where I got hit when I didn't think I should have got hit. So good job on that.
The Bad Points - The graphics were kind of just plain and boring. I wouldn't have minded some nice backgrounds and better colors to make the game easier on the eyes.
~ Story/Content ~
The Good Points - The difficulty was pretty good. You almost had it perfect where it wasn't really too hard, but it wasn't too easy either. (almost)
The Bad Points - It's not very fun or exciting just to have one plain level like this where you don't have much to choose from or you don't have much content. Just the same old same old. Very unoriginal.
~ Things To Make It Better ~
Levels - I rather see a game like this with levels so you have a certain amount of points you need to gather before the level is over and you can go to the next level.
Upgrades - Both in between levels and during levels would be nice. You could have things such as:
1) Shields - Where you can take a couple of hits from the enemy before you can die. You can have these to buy between levels in a store and/or have random ones pop up during the game.
2) Invisibility - Make it so this only lasts for a certain amount of time, but while it's lasting the circles that follow you will just stop for awhile. Have this primarily pop up randomly during the level.
3) Slow Down - This can make it so the circles are slow for a small amount of time. Only have something like this during the level.
4) Invisibility - Make it so that for a short amount of time no matter what you run into you won't die.
Money system - Of course with a store you would have some sort of money system. This could be done by having random "money" pop up during the level that you could collect if you passed over it.
~ Audio ~
The Good Points - The song was a decent choice and was a good choice for a game that was fast pace with action.
The Bad Points - The only thing I can think of is how it replayed every time that you tried the game again. It would be much better if you took something like this and fixed it, which I noticed you said you did in number 2.
If you added levels and stores - You could add audio for the different levels of the game so you could have a variety of music for the players. Also, a nice relaxing song during the store so people could calm down after the stressful level.
~ Overall ~
It really does need a lot of things to make it better. Perhaps the stuff that I recommended would be too hard for you to program, but without things like this it's just your every day avoid game, which there already is 1,000's of them here. If your going to make one and want to get a good score/rating then you have to do something to make them original in some way.
I hope that my review gave you some ideas on how to make the game better in the future. Good luck and keep working.