It's a very well-polished little puzzle game, and it's pretty fun to play overall.
I had some troubles comprehending connections going through other connections (after leaving portals), and don't even know whether it was intended at all. Also, I think the timer (apart from the last level) is just annoying and doesn't add any sense of real challenge (unlike the last level which I interpret is about discovering the solution and executing it flawlessly) as the player will either discover the solution on their first try, or eventually stumble onto it, and then the timer is generally just too generous.
Further it might just have been my sloppy controls, but undoing lines was just way too hard for me to the point that restarting the whole level became easier. Even further, sometimes selection would not work, and I'd have to click again to start a line.
Overall, it was a pretty fun experience despite the minor hiccups.