I really liked this concept of an outsider inhabiting old games, and the gameplay not just becoming what the original game's gameplay was -- the Tetris level in particular is a clever bit of gameplay building on that game (though I got stuck in a spot I couldn't get out of).
However, the game is VERY buggy, so it's quite difficult to play. I got stuck on enemies and walls all the time, though I found that I could often wriggle my way out of it with some button mashing. I'd also have liked a character controller that was a bit more snappy. The slippy movement got me into trouble a few times.
Also, was I supposed to be able to get onto the blocks in the Mario level? I tried but couldn't jump high enough, even though there was a mushroom on top of a ? block.