I only had one problem with the game: the difficulty curve. Some levels are ridicoulosly easy while others are nearly impossible. Anyways, it's a nice game, with a nice "push "idea.
Congratulations for your first game! You have to work a little more on it (more types of enemies, more powerups, moving backgrounds, etc.) and it will be a great game!
Great one, I love these kind of games.
The only problem is that it gets too laggy on some levels (especially on level 20), and it makes these levels pretty annoying (and sloooow).