Interesting thread...
When I read the books for real (I tried unsuccessfully once...it's a long story that I've told eslewhere on this site before), it was about a month before the movie came out. So I think that my preceptions were slightly colored by the trailer, though they're more different from the movie than they are like it.
My own imaginings are pretty difficult to describe since they are so deeply internal. The closest I can come is to say that my imagination must have taken the path of least resistance--that is, blended certain elements that I liked from the movies with a basic picture that was my own from the books. Sometimes, Middle-earth and its inhabitants look and sound similar to how they do in the movies, and sometimes they don't.
I find Frodo to have been the most deeply affected by the trailer. He still doesn't look the way he does in the movies, but his coloring and facial structure is similar at least. What can I say? I was 13 years old and I found him attractive. Of course that was going to color my imaginings a bit.
The Hobbits don't have those accents in my mind. In fact, generally they don't have accents at all, since I can't passibly replicate any accent other than my own, more's the pity, and I'm too lazy to try to imagine accents anyway. They don't "sound" american, per se, but they sure don't sound like they're out of the English countryside or Scotland or what have you. To describe their looks would be difficult, but they all are roughly the size and shape of Movie!Sam in my head, though that's perhaps still trimmer than Tolkien intended.
I find that there are fault lines in my imagination, though, between movie and book, and when there is a departure from the book, my imagination shifts. The Old Forest looks pretty much the same now as it did in November 2001...and the Hobbits all look a little...different when they're in there. It's consequently one of my favorite parts to read now.
Something similar is true of Faramir. He doesn't look much like his movie self (even though I like David Wenham and think he's a good actor who did the best he could with a faulty script--not so much the departure from the book, even, as that they seem to have treated Faramir with a Denethoresque level of neglect and carelessness). Faramir is taller in my imagination and resembles Aragorn more than he does Boromir.
As for who's closest in terms of book and movie? Ian McKellen's Gandalf looks about the same as my own did. Movie!Aragorn is also fairly similar to my imagination.