It was hit or miss with me.
Aragorn - I imagined him as somewhat older-looking, or more rugged. Still, I think the casting of the part has grown on me.
Boromir - I liked the casting for this part. Sean Bean is very close to how I visualized Boromir, and played the part well.
the Hobbits - in general, the Hobbits look like how I think of them - essentially, small beings with human-adult proportions. The casting was so-so for the leads. Sam didn't look much at all like I visualized him, but Sean Astin actually seems to fit the part. Merry and Pippin look too interchangeable to me; I always visualized Merry as darker-haired and a bit larger than Pippin, and more mature-looking.
Gandalf - I'm astounded by how Ian McKellan made this part his own. His portrayal of Gandalf is spot-on to me.
Gimli - I think they could have simply let Rhys-Davies go without the prosthetics, because his real face looks a lot like how I pictured Gimli.
Legolas and the rest of the elves - I always pictured elves as supernaturally beautiful, men and women alike. The elves in the film just didn't seem as ethereal those in the book. I don't know how one could have achieved that anyway in a live-action film. I do like the casting of the role of Legolas, though, and I liked the performance.
Galadriel - Cate Blanchett is perfect for this role. Still, I didn't think she quite pulled off the authority and gravitas needed for the role.
Arwen - well, I'm not a Liv Tyler fan. I don't think much of her acting, and I visualized Arwen much differently.
Elrond - Interesting choice of actors. I think this casting choice worked.
Saruman - My visualization of him was always kind of a generic old-wizard type. Christopher Lee is now embedded in my mind as the perfect Saruman.
the Orcs - I always imagined them as more bestial, a lot like how they are shown in the Monster Manual. I think they worked well in the film, though.
Trolls - meh. I wasn't all that impessed with the troll. I imagined them a lot differently.
The Black Riders - Very good, except when Frodo sees them in the spirit world. I imagine them as gaunt, tortured, desperate-eyed men, once regal, with steely eyes and haggard (to use a word Tolkien used) faces. To be honest, I thought they looked kind of dumb in the film - in the spirit world, that is.
the Balrog - very good interpretation of the creature. Liked it a lot.
Overall, the film's landscapes and vistas, and the ruins and buildings, all gave Middle Earth a grandeur that came close to how I visualized them. Very nice. Moria was as vast and dark as I'd imagined.
Overall, pretty good.