I saw it opening night, and the only regret I have is that there wasn't a midnight showing nearby
About the time, I didn't even notice. If I didn't know better before I went into the theatre, I would have sworn it was only an hour long.
* Some stuff below could be considered spoilers, so shield your eyes if you don't want to see those
*
It follows the book fairly closely, leaving only a few main parts out (most notably, Tom Bombadil and the barrow-wight), and changing the order a bit to make it a stand-alone movie. People who had not read the books would be wondering, "What the heck is going on?", had the introduction not told of the 3 Elf Rings of Power, 7 Dwarf Rings of Power, 9 Man Rings of Power, and the forging of the One Ring by Sauron.
The increased role of Arwen (Liv Tyler) wasn't as bad as I thought it would be. She is barely mentioned in the first book, only saying a few words and being seen with Aragorn/Strider by Frodo in Rivendell. You don't really find out what that's all about until the end of Return of the King, where Aragorn marries Arwen. In the movie, the only thing Arwen does is carry Frodo to Rivendell after his wound on Weathertop.
Also something that I noticed is that it seems Saruman has become a servant to Sauron in the movie, where in the book he only feigns cooperation while conspiring to replace him as the Dark Lord. It may turn out that way in the Two Towers movie, but it doesn't seem like it in the Fellowship of the Ring.
The parts that are sticking the most in my mind are the two "scenes" from the book that I was most anticipating to see in the movie: the Balrog of Moria, and Boromir's "Last Stand" (technically should be in Two Towers, but I don't care too much about that). I wasn't disappointed, both were done well.
Only other thing I can think to say of the movie, is that there wasn't nearly as much singing as there is in the book (which is about once every three or four pages on average, IIRC). That's probably a good thing, as it would get annoying after a while
Go see it!