The first season is amazing. Fast paced, suspenseful, well constructed. The high point is probably the Watanagashi arc and its answer arc, but the last bit with unlimited cleaver works was pretty good too.
The second season is mediocre. It slowed down a lot, the lack of question arcs meant no suspenseful atmosphere, K1 turned into a typical shonen hero, and there was too much "believe in yourself" shit. Also, far less gore. But its still worth watching.