One Major Flaw
First, I'd like to say the style of this game is well-done. Controls are simple and you can tap or hold down the space bar to toggle viewing and dragging. The graphics look more programmed than drawn, so maybe faint texturing would have been nice on the sides, but as it is it's fine. It probably would have looked pixelly any other way.
The background and music were a bit too bland for my taste, though; you should have had something relaxing drifting across the background or something, and the music was a little too slow for my taste. Perhaps you could have used multiple audio tracks? I'm a little sick of hearing the same loop over and over.
The gameplay was pretty fun but some levels were damn frustrating. Maybe a timer, a points system, etc. could be used so that people could buy additional hints, or maybe you could have a hint popup specific to how many pieces you had remaining. I also hated that the blocks are solid; sometimes I'd try to move a piece only to find out I couldn't, but the innermost parts of a series of blocks was so dense I couldn't tell what piece it was hitting. Transparent blocks would have been much easier.
Other than that I really liked this game, but it needs some work. And maybe more puzzles; twenty is usually the bare minimum amount of levels you want in a game.