This game is absolutely fantastic and I had a wonderful time playing it. I finished it over the course of three days. I estimate I was actively playing or thinking about the puzzles for 4 - 6 hours in total.
The puzzle I was stuck at for the longest was #5, with #13 coming in second, and #11/#15 tied for third. I found #7 to be a great moment where I transitioned from figuring out how everything works to competently using the tools.
My only gripe is that my progress was not saved when I exited and came back, but that just gave me an opportunity to re-solve some puzzles.