I read the book and I'm very much looking forward to seeing it. Jackson had warned that those expecting everything to be true to the books would probably be disappointed with the RoTK. He made some lame justification about keeping the length of the 3rd movie down required some "artistic" license, combining characters and even creating a new character to cover some material. I'm sure it will be good.
Just be sure to use the bathroom before the movie starts and limit consumption... I clocked it at about 3 hours 15 minutes... Went to the 11:15pm and got out of the theater at 2:45 am (20 minutes of previews/commercials, but they started them 5 minutes early at least).
I always go to see these on Christmas day, a "tradition" since the first film. I am currently watching "Fellowship-Extended Ed" on my 51" widescreen HDTV to be followed by "The Two T's-Extended Ed" just to get me in the mood.
Two co-workers went to the "marathon" and loved it (ROTK). One said it was the best, the other thought Fellowship was the best. Interesting to contemplate what I will think is the best. I guess one should view all films as a whole. Of the two I have seen, Fellowship has the lead.