Very good, I really liked it :)
it was rather addicting, If I didn't have to get some sleep soon I'd probably end up playing it all night :P
I had an idea, what if you created obsticles to work around in future levels? For example: a rock that would cause your box to be unable to fill in? Just a thought.
Also It would have been nice to be able to move accross areas that you've already filled in. It was a little annoying having to retrace your steps to move around some areas.
Also I think it would be cool if you could move diagonal angles too.
Anyway, great game, hope you implement my suggestions :)
But in any case, good luck on your next one!