I just finished watching the new 4k blu-rays as well.
I would say FOTR is an improvement over the old blu-ray and for sure the best looking of the 3 movies. I really liked the new grade apart from The Shire and the council scenes.
TTT and ROTK I still prefer the old blu-rays. I was actually shocked by how bad ROTK looks. Like Jay above me I would have to agree that ROTK is one of the worst 4K discs I have seen.