Speaking as a huge fan of the comics, I was very disappointed. It wasn't terrible, but it was a pretty lame adaptation, reminding me of Last Airbender- well, not that bad
Hal Jordan is the ultimate cocky badass so seeing him as a self-doubting wuss really, really grated on my nerves. Hector Hammond is far and away the silliest and lamest villain in the GL mythos so why they decided to use him as the first movie villain is beyond me
The CGI (I saw it in 2D) was generally terrible and fake. Was it really necessary to CGI Hal's uniform at all times? It looked so fake. Just put a damn practical costume on him for crissakes. I wonder how many millions of dollars they wasted doing that... Parallax looked goofy as well. His "face" looked like some lame generic brand Halloween mask, and his full form looked like a massive cloud of brown turds sliding around. The Guardians didn't look that hot either. I'm not crazy with their redesign of Oa either but I'll admit it didn't look fake or anything.
I don't know why they used this story instead of something actually from the comics. "Secret Origin" would have been a workable storyline. A story that actually built on the relationship between Hal and Sinestro, showing them working together and not just sharing a couple scenes together. And they missed that classic line where Sinestro first meets him and explains how wonderful he is and how everyone follows his orders and Hal casually says "yeah, that's not gonna happen with me", something like that.
Maybe I blanked out for a moment there but someone explain something to me. The GLC is at war with Parallax. They sent a strike team to take him out but they fail. They're building a yellow ring to use against it. Hal declares he's going to fight Parallax himself. Nobody goes to help him. But once he's won, they all show up to save his butt. So were they literally just squatting outside the solar system watching him get killed and doing nothing? Pretty brave, Sinestro.
But I think the worst thing about the movie is that it was boring
. Hardly anything happens until the ending. But I say the same thing about the Spider-Man movies and everybody loves those :-p