I know this is an old thread, but it is in my top 3, along with Anna K and Tess, so I wanted to contribute.
I first read GWTW when I was 17 - I'm 68 - and re read it when I was 25, and 48! Time to do it again, I think, on Kindle this time.
I saw the film when I was 18, and didn't think much of it, compared to the book, but I have since seen the film about 4 times and love it!
It is a truly magical book. If anyone reading this hasn't read it - please do!