I wanted to finish the book first, but, well, I got dragged along against my will. I don't think Mom took my comments about hordes of squealing teenage girls seriously until we got into the theater. I could almost hear the "oh shit" thoughts descending. And there was squealing, and cries of "I love you Edward!" And other such nonsense.
But squealing hordes aside, it wasn't a bad movie. The dialogue and pacing was much improved, and I didn't actually have to deal with Meyer's purple prose. On it's own, it's a decent movie. Probably not something I'd want to shell out money on for a DVD, but worth bittorrenting, or seeing once and never paying any attention to again. In comparison to the book, it was a vast improvement. There were a number of moments in the movie that were hilarious - that I don't think were intended to be funny - and some characters got expanded roles, while others were pretty much squeezed out to save time, and there were one or two scenes in the movie that got added or altered, or combined with other scenes.
I kinda thought they'd put a little more effort into the special effects budget though. Seriously. At first I thought there'd been an accident and Crouching Tiger Hidden Dragon got spliced on the reels.