Eh, it has a concept that differentiates itself from The Matrix enough to be considered clever, two or three clever action scenes, it looks nice, has a decent ending and, well, that's enough to be considered four stars in my book.
The first entire half of the film is dreary exposition, the script is uninteresting, the characters recycled archetypes, one of the protagonists is unintentionally infuriating, the acting passable and the concept confused and inconsistent (despite being clever).
Scott Pilgrim, on the other hand, is like Speed Racer done right. That's not fair, really, because Speed Racer was just a shitty Lazytown adaptation, but its bright colours made Blu-ray collectors climb up their own arses and set up camp. It's like Kick-Ass done right. That's not fair either, because SP was an adaptation of a much better book and didn't have a "superheroes in real life" premise that it unceremoniously and inexplicably dumped halfway through - a la Hancock. It's like Kick-Ass meets Speed Racer, if either of those films were good, meets Shaun of the Dead.
Pilgrim was a brilliantly eclectic, soon-to-be-cult film. Its action-scenes-as-videogames idea was original, expertly executed and, fortunately, the film didn't gather enough popularity for it to be repeated ad nausuem. Its humour was very good but - like the film overall - was also very eclectic. Cera's casting was good but there were likely better choices who weren't box office draws. The casting of every other character was bang on and, to momentarily contrast with Inception, were just as two-dimensional. However, in this case, that was intentional for humour and they weren't all tired archetypes.
In short, unlike most mainstream films, it has the balls to put itself out there. As Elbert Hubbart said - and Christopher Nolan took onboard - "To avoid criticism, do nothing, say nothing, be nothing."