This is definitely something. I didn't expect it at first, but I ended up getting hooked.
The presentation is pixelated and nice. I really like the retro style it goes for. As a throwback to the birth of video gaming, we have some gritty sound effects and no music, but I think that's fitting.
Gameplay wise, I didn't realize what was going on at first. It took me a while to understand that the tile we "move" is actuallly the empty space on the playing board, and we're sliding tiles around to form tetraminoes. The number of moves we're given is limited, and we have to be extra careful to not waste them by moving tiles to places where they don't fit, resulting in a bit of a mind challenge at times.
The game has two modes, as mentioned on the description, and I feel like marathon mode makes more sense. I mean, twenty mode is literally bound to end in a very small amount of moves, even if you're as careful as you can be, while marathon mode rewards you with more playing time whenever you do something right. That's exactly how old puzzle games like Tetris worked: if you cleared a line, you got more space to work with and, therefore, more playing time.
When it comes down to it, though, this game suffers from the same problem as many games of its type: the novelty wears off quickly, and the game gets a bit old. It's just too empty and there's not really that much going on. In general, though, this is solid, and definitely a nice little time killer.
So, in general, a pretty nice little puzzle game.