As an aside often when someone mentions a film they have watched I will ask if they've read the book and the answer is invariably no.
For most people watching a film is an order of magnitude less of a time commitment than reading a book. It's much easier to see a lot more of them. And some films are actually good (and many of them aren't based on books).
Plus watching films is often a social activity, while reading books usually isn't. This tends to lead to different priorities.
I find this really sad as the book will nearly always take the story to a whole new level that film can never achieve.
That's why seeing the film first is the good way round. Less disappointing that way.
It's also, for some people, a gateway to reading. If you enjoy a film that was based on a book, chances are high that you'll get even more out of the original novel, or that you'll enjoy other works by the same author. That can be important if you find reading difficult (or just plain slow). My brother was a very reluctant reader (I suspect he's mildly dyslexic), until after watching
The Eagle Has Landed (he was a WW2 buff at a formative age) my mum pointed out that there were a several of Jack Higgins books on one of the shelves upstairs. I'm sure
Harry Potter has done similar for the current generation of reluctant readers.
I think we're still some way away from the Internet having a negative effect on literacy (indeed, I can think of a couple of adults I know for whom it's had an extremely positive effect). While video proliferates, it's still fundamentally a text-based medium. I'll also suggest that there's a lot more to reading than novels. For a long time I didn't have the attention span for reading fiction, but I would spend many hours a day reading everything from realtime chat, through random bollocks on newsgroups, intelligent blog posts about all manner of subjects, textbooks, technical references, scientific papers and random wiksand. It's all good.