ive nearly finished the first twilight book and didnt know whether to start the new moon bookor to watch the film first. I watched twilight before the book but had family around so didnt really watch it properly although i did think it was pretty crap compared to the book.
I always read a book before watching the film if it something that interests me (i.e. not applicable for LOTR as I wouldn't read those books).
I have never once found a film to be better than the book it comes from and if I watched the film first it would totally ruin the book for me - spoil the plot etc, yet a book rarely spoils the film (as it is usually crap in comparison anyway!).
But this is just me, some of my friends are the opposite - although I find that these are the ones that are not big readers and don't get immersed in the world created by a book