Well, Eragon's a given...
Bridge to Terabithia - Loved the book and just seeing the trailer warned me against watching the movie.
Inkheart - I read the books, loved the characters and pretty much everything about it. I randomnly noticed the movie a store and ignored my friends' warnings and watched it. I really didn't know whether to laugh at it or claw at my eyes, but that's just me.
Vampire's Assistant - Does absolutely NO justice to the Darren Shan books, in my opinion.
Ella Enchanted - all my reasons have already been said by everyone else

Those're the ones that spring to mind, off the top of my head. I don't really have an opinion on the HP movies, or Percy Jackson for that matter.

And although I hate it, I will say one thing that I loved about seeing the Twilight movie. It wasn't anything to do with the movie itself, but the fact that when Edward first went sparkly, my friend Justin shouted out "SOMEBODY WENT TO JARED!" That alone made it worthwhile for me. mrgreen And I was dragged to see that movie by my best friend who is an extreme twilight hater (I don't know why she wanted to see it, don't ask) but at the end of the movie while she was calling her mum for a ride she said "Yeah TWILIGHT SUCKS" really loud just as a group of tween fans walked by. The expressions on their faces were hilarious and I vaguely wondered if we were gonna get jumped by a bunch of 12 year old girls. Hilariously, that group of little girls was standing and waitin' around outside the door to the screen we were in and we had to sneak around them. These 2 expierences were the only good things that came out of seeing the Twilight movie. Other than that... crap.

Oh and I totally agree with Real Horrorshow Groodies, up there! A Scanner Darkly was amazing!