I used to play with tanagram puzzles as a kid. I loved them, and this game is basically a tanagram puzzle taken to the next level. The magnet gimmick is super creative, it's really simple but leaves lots of room for challenging and creative puzzles. The undo button is also a lifesaver, so I appreciate that you put that in. The graphics and music go well together, and I like that the outline of the empty space shows when there's a piece overlapping it.
The only gripe I had with the game was the difficulty curve. I actually found some of the earlier levels to be harder than the later ones. I would've added in some more easy levels at the beginning. The game overall is very challenging since there's two sources of difficulty: figuring out how the pieces should fit, and actually moving the pieces around. However, this is overall a very tiny nitpick - I personally enjoyed the challenge and spent a lot of time on the harder puzzles.
Super great job overall. I love the mechanic and I managed to beat all of the puzzles.