A few days ago I completed all 600 levels. Crazy, I know. Having done thousands of these types of topology puzzles, I found most of the levels not too hard. But every so often a level would have me stumped for days and weeks. The last 100 levels I did in about 2 hours, on a plane ride.
So yes, I really like this game. However, at the moment, it's not possible to go back and redo any level at all. Unless I delete the program & then reinstall it.
Some other good topology games:
Lines Numeric Fun