A very entertaining movie. A successful translation of the rest of the story to film. I'll leave the detailed discussion of the artistic and structural merits -- as well as its fidelity to Tolkien's work -- to others.
A few random comments:
I thought the opening worked well, a good way to get the audience back into Middle Earth.
Although the character of Denethor was a chump -- does Peter Jackson have something against lines of stewards or something? -- the scene of him eating while Pippin sings and Faramir's men ride to slaughter was extremely effective. (One of the artier bits of the whole series, I think.)
I still don't like slow motion shots. Especially slow motion shots of people smiling.
The battle scenes worked for me. There's nothing else that satisfies in quite the same way as the sight of an orc looking worriedly at six thousand charging horsemen. (Although considering what Legolas, a Wood Elf, does, it's hard to believe every single High Elf archer was killed at Helm's Deep. (And if they weren't killed, where'd they go? The West?))
Generally speaking, I approve of the changes Jackson & Co. made. The original story is better, but dealing with what was left out would have added another hour to the movie.
I'm not sure, though, about having Arwen rescue Frodo from Shelob.
(Oh, and the Dark Lord comes off looking like a big cartoon lighthouse, whose two lines are, "Hmmmm..." and, "Huh?")