Fairly common opinion, I think. Personally I really enjoy Duke's Archives and the DLC area, but nowhere near as much as the trip from Undead Asylum to Anor Londo.
I think the dense experience of Sen's Fortress into Anor Londo culminating in a hard-ass boss fight was unbeatable. Nowhere to go but down from that.
I felt the main problem the later stages had is they started you off in interesting environments, and each level you work toward less interesting places.
For example,
Catacombs > Tomb of Giants
Demon Ruins> Bed of Chaos
New Londo> The Void
Duke's Archives> Crystal Caves.
The sections before that were non-stop awesome section to awesome section that never dragged on too long.
Still, it's my favorite game to come out in the last two years. I hope the next one knocks it out the park.