Awesome game.
First off, my hat's off to you guys. It's very addicting. To other users who have said you have to guess, I don't think that's the case, I've done every puzzle except the first 10 of the "extreme pack" and haven't had to guess even once. Instead, you might want to try flexing that brain muscle a little more.
Now, my problems with the game are this: the art is awful. And by "art" I mean the reward at the end for solving the puzzle. In most cases, the resulting image is more pixelated than the puzzle itself. I had to take a few seconds and savor my work before filling in the last box and my hard work was whisked away, replaced by giant black blocks.
One extra tweak might be to provide a modifier key to fill in all the boxes between two clicks with "X"s. Having to drag the mouse (or trackpad, even worse) along 25 squares is a small pain in the ass. Two clicks would have been a good idea. Something most people wouldn't even notice but power users would eat right up.