I'm gonna rant a little bit here. I'll try to avoid being spoiler-y about he books but I'm going in assuming peeps have seen the movie.
I thought SCOTT PILGRIM vs THE WORLD was bad though, and NOT because "it was different from the book!" It really wasn't THAT different, at least superficially. It hit the big story beats, sure. It did a fine job of keeping a lot of the funny jokes and silly hipster spirit but at its core it completely whiffed on the point of the story: Scott Pilgrim is a douchebag in sore need of some growing up.
My biggest problem is how much the movie let Scott off the hook for his dick behavior. Sure the characters roll their eyes at Scott a lot (ditto the book), but as the books progressed forward and you learned more about Scott's romantic history (with Kim, Envy, and Lisa, whose roles were reduced to non-existent in the movie) and how he wasn't just an endearing but absentminded doofus, he was actually kind of horrible. The books beautifully show everything bottoming out in slow motion, because of his stubborn refusal to acknowledge that he's being self centered even though he does genuinely care about those around him. Whether he ends up with Ramona or ANYONE not was not the point. The fighting the evil exes, that ALSO was not the point.
The movie kind of chucks all that out the window. Michael Cera plays Michael Cera, lovable awkward doofus, and ends with the message that TRUE LOVE CONQUERS ALL blaring with bright colors and indie rock. It ends up being a personal victory without any of the interesting personal struggle.
The reason I picked Scott and Knives is because it's the difference in the book and the movie in a nutshell. Scott cheats on Knives and Ramona with each other. Knives in the book is shown to be immature (for sure), but she's also 17 and struggling with a lot of sadness/longing/frustration with this older jerk who played her emotionally and then unceremoniously dumped her when she stopped being the most interesting thing to him this very minute. In the movie she's just this cute/annoying groupie who ends up being as feasible a love interest as Ramona in the end.