To me it is an incredible shame to see the movies before the books when it comes to LOTR. The movies are okay, but it is nine hours of entertainment, then it is over. Now the book is spoiled. Reading the book first you get all the enjoyment and excitment of the book, then when your finished you can watch the movies and still enjoy them just the same. The movies also have the very unfortunate side effect of robbing the free reign of your imagination regarding the characters, places and events of the book. If see the movie first Frodo will forever be Elijah Wood when reading the books. Anyone who has read the books first will have came up with their own look and feel of all the characters and places in the books. Imagining the world is IMHO one of the greatest parts of reading the books. Just the fact that Dayv said he recommends seeing the movies first seems to indicate that he himself saw the movies first.

I am a strong believer that films ruin books, but books don't ruin films. There are some exceptions to this, especially if the movie is better than the book on which it is based. Planet of the Apes is one of those I would highly recommend seeing the movie first. With things like Fight Club it does not matter because the book and the movie are very nearly identical, no mater which you see/read first, you will feel like you have read/seen the other before.