Very hard but very rewarding. I think the weaker puzzles were the ones with heavy "traditional" platforming, like the spike room and many of the laser rooms. They aren't bad, just something that could be in any game. That said, many of the puzzles are very clever and well designed, I had many "aha!" moments with this one. I didn't finish it, but I did play it for 40 minutes straight, so that should say something!