There are 4 levels of difficulty. The easiest uses a 4x4 grid, while the hardest uses a 7x7 grid. I've solved puzzles at each difficulty level.
The puzzles seem to be randomly generated each time. Which is good and bad. Good, if you want to keep playing this game forever. Bad, if you're the type that wants a set of 50 or 100 levels and solve them all with a sense of accomplishment.
I like this concept. However, there are multiple solutions, which make the expert levels not that hard.