Definitely agree on Starship Troopers, Battlefield Earth, Jurrasic Park, and I admit to being pretty peeved at Peter Jackson for Two Towers. It was one thing to omit stuff for time reasons in Fellowship of the Ring, but he was making **** up in TT. Running Man (the movie) is basically a completely different story from Running Man (the book), so I enjoy the movie just for the cheesy one-liners that Arnold throws out there. I think they could have done a much better job with The General's Daughter, too. Maybe it's just that Travolta really is a ****ty actor though.
There have been a couple of movies that they actually did a decent job with when converting from books. The ones that I can think of off the top of my head are The Last of the Mohicans, The Shawshank Redemption, and The Green Mile. There were minor deviations in all of them, of course, but I still think they followed the book pretty closely.