I quite enjoyed the game - it's simplicity was quite charming but it wasn't too easy, either, and it was quite fun.
The music was excellent throughout, especially the boss theme, so you get full marks for that.
However, there are too few levels, and the limited graphics do make them look a little dull and bland.
The controls for the wall jump were disastrous - other than just holding up and letting the auto-jump feature do its work, I could never really utilise the wall jump for anything, and it frequently became an annoyance as I ended up latching onto walls and then leaping off of them again when I was just trying to ascend some steps or ledges. One level in particular required frustratingly precise movements in order to avoid launching oneself into a bomb turret.
The boss was no real challenge at all, and indeed suffered from a bug in the second phase - if you stand in the gap in the centre, and shoot the landing blocks instead of the ship, the whole boss still takes damage.
I think a lot more could be done with the game, but it's not by any means a bad start.