My best friend said ROTS was his favorite Star Wars film right after he got out of the theater.
A couple of years later he started noticing all of the things horribly wrong with it, and that was the same period I "sorta" liked it... and one time I told him to come watch ROTS with me since I was in town, and he grew disgusted. I asked "What don't you like about it?"
He said: "There's nothing to like."
We did try to watch it, and only sat through a few minutes. Later we just decided to go play Dungeons & Dragons instead.
Basically Darth Vader is his favorite villain since childhood (mine also), and TPM and AOTC got him hyped up to see the dramatic transformation. He felt Darth Vader was put to shame with ROTS. Instead of being a badass, he was still the childish Anakin from AOTC.
So yeah, I know what you mean.
I almost get the impression that "Star Wars: The Clone Wars" is Lucas' attempt to put in a good name for himself after the travesty that was the prequel trilogy. He knows they're bad, and I see the new series as a cloaked attempt to redeem himself.