I just got back from the midnight showing. But first, some cred. I started reading the books around two years ago, right before the fifth volume came out. I can't count how many times I've reread them. What follows contain minor spoilers that only really make sense if you've read the books in the first place.
So the flaws. It's not so much that it deviated the comic, it's that the movie mutated the comic. There would be scenes that would be the same except one of the characters is different. For example, most of the elements of Romona vs. Envy are now in Romona vs. Roxy. Or scenes that are the same except they're not quite in the same chronological order, like Romona vs. Knives. And of course, there were things left out for time, including Honest Ed's, which was one of the most delightfully surreal parts of the comic. And there were other things that were just overly simplified, like Kyle and Ken Katayanagi.
And now the good parts. EVERYTHING. Oh. Em. Gee. Best comic adaptation I've ever seen. Just amazingly awesome. They just nailed most of the characters. Michael Cera makes a much better Scott Pilgrim than I thought he would. Wallace Wells and Kim Pine and Knives Chau and Ramona Flowers and Gideon Graves and Todd Ingram were absolutely perfect. Steven Stills, not so much. Too goofy. But still awesome. The soundtrack was kickass, and the visuals were clever as hell.
It boils down to, are you alive? Then go see the movie.