It depends which I come across first. I think I do tend to read the book first, which I did with Harry Potter, because I like to get all of the detail in my mind first - I think this enhances the experience I have when I watch the film, as I'm able to fill in the gaps. I think I get more out of the film if I already know the whole story.
Films are criticised for leaving too much detail out, but it's usually not possible for every aspect of a story to be included because of budget and time constraints. But I don't really think that the purpose of the film is to tell the whole story necessarily (I don't see it that way); I see the film as a chance to have the book brought to life, in just a small way. It doesn't really compare to what I see in my imagination as I read, but not everyone reads in the same way - imagine, some people don't see pictures when they read!
I don't tend to find the film disappointing if I've read the book first, because I just enjoy seeing a director's interpretation of a story that I have loved. (Although, I was hugely disappointed with the screen adaptation of 'Eragon' - it was pretty dire!)
Watching the film first and then reading the book is probably the more logical way to do it. You get a taste for the story, and then read the book to learn more. Kind of like saving the best to last.
So, now that I've talked that out for myself, I've come to the conclusion that it doesn't really make a difference which way around you experience the book and the film!