My short answer: I'm not a stickler either way. There are some cases where I planned to see the movie version but wasn't all that interested in the book, and the movie made me want to go back and read the book anyway. Sometimes I like the movie better - or in the case of The Lord of the Rings, the movies, despite the heresy of that position. Sometimes I still prefer the book, in part because of the parts that don't make it into the movie. Sometimes I will dread the movie adaptation of a book I loved and be won over despite myself, as happened with Wonder Boys. Sometimes seeing a favorite book turned into a movie actually helps me visualize parts of the story that I had trouble imagining on my own. Sometimes I didn't even realize a movie was based on a book, and if I liked the film, I've found something new I want to read.
I think one thing that factors into how one answers this question is one's feelings about "spoilers" - how adamant one is about not wanting to know about the story in advance. That used to bother me, but it hasn't for awhile, especially since we began recording TV shows on our DVR to watch later; I still read the online recaps even if I haven't seen the episode yet. I like seeing how it plays out even if I already know what happens, and since some of my favorite shows, like Lost and Life on Mars, have complicated plotlines, it actually helps me to have some advance intel.
For me, though, I think the bigger factor is seeing "the book" and "the movie" as related but different, and taking each on its own merits. In a Sunday Salon post earlier this week, Literary Feline talked about how reading, and falling in love with, the novel The Princess Bride made her appreciate the movie - which she had seen numerous times before she got around to reading the book, by the way - more. I saw the movie first too, and I'll always love it more than the book, but I appreciate them both as the separate works they are. Especially when the movie is "based on" or "inspired by" a book rather than "adapted for the screen," I think holding it apart from the source material probably causes a lot less frustration over what the movie "got wrong."
This is actually a topic that comes up pretty often around our house. My stepchildren's mother has a rule that the kids cannot see a movie based on a book unless they have read the book first, and it's a rule that has to be followed in both households. As a mother and a reader, I understand this thinking - the main concern is that the kids will feel like they already know the story and won't want to read the book, and we don't want to discourage reading - but I don't really agree, and their father agrees even less. The kids in this case are already readers, and we don't think they'll start ignoring books.
On the other hand, we wonder if making the book a prerequisite to seeing the movie has taken some of the fun out of reading, or pushed them to read books they weren't ready for just because they wanted to see the movie. Our nine-year-old has already read Harry Potter and the Goblet of Fire on his own so that he'd be permitted to watch the movie - and not to take away from that accomplishment, but I don't think that the material in the later Harry Potter books or movies is really appropriate for kids that young. Still, "first the book, then the movie" will probably be a fact of life for him and his sister for a while longer, which means he's still got at least a year to read The Hobbit before that movie comes out.
As long as I get something enjoyable or interesting out of each, it doesn't really matter to me whether I read the book or see the movie first. What do you think - book first, movie first, or do you just want to be entertained?