Spoiler alert! If you still want to see Green Lantern, don't read this.
Even after I downgraded my expectations for the Green Lantern movie (based on all the negative reviews, though I still held out some hope as a comic book geek that maybe it was too comic booky for everyone else) I still found myself disappointed. Often when a comic book movie disappoints, it's because the director seems to be completely ignorant of its source material. Elektra, for example. But that's not the problem here- save some minor tweaks for streamlining and simplicity's sake, the comic books, and specifically Geoff Johns' excellent version, are more or less faithfully transferred to the big screen. Maybe that's the problem- Hector Hammond with his oversized brain looks silly in "real life" and as a villain he fails because the poor guy was an innocent victim of circumstances he never asked to be involved in. He's quite a sympathetic character, actually (in the comics he has no redeeming features). Hal Jordan isn't even as sympathetic as poor old Hector. Hal's kind of an ass, actually. Not a charming ass like Tony Stark, just a kid who never grew up and doesn't know how to clean up the messes he makes. Maybe ass is too harsh, since I did like him, and I liked Ryan Reynolds fine.
 Parallax? Well, kind of lame. Better than Fantastic Four 2's Galactus in that he was more than a giant cloud- but not much more. And of course it stretches credulity that the entire Green Lantern Corps would be "He's too tough let's let him win" and then when Hal is like, "I'll fight him all by myself even though I am the greenest of Green Lanterns. Everything will be okay, you guys just chill on Oa!" I did like that he basically punched Parallax in the face, though. Nice.
Sinestro was the best part of the movie, I thought. The actor classed up the joint with his performance. Unfortunately, the final scene where he puts on the yellow ring doesn't really work because there is absolutely no reason for him to do so. In the comics he has a very complicated and nuanced reason for creating the Sinestro Corps and going to war with the Green Lanterns. Here it looks like he did it for the hell of it. I stilll hope they make a sequel, though. A war between the Corps would be pretty cool on the big screen. Maybe.