I think at this point, most everyone is aware of Lantern's premise. Guy is chosen, guy looks hot, girl likes guy, alien hates guy, guy fights for alien, blah blah blah. I'll try to not repeat what smarter people have said better than I would.
First and foremost, I really liked this movie. Don't let anything I say after this tell you otherwise.
My bad: We all saw that first trailer and cringed. There was a bit too much Cornball Reynolds for my version of Hal Jordan. And on that train, there was quite a bit of cheese dialogue throughout the whole movie in general. Visually, everything looked quite amazing but the end battle was very lackluster. Very short, very simple. Get in and get out. I wanted a lot of shit flying around and blowing up. Epic standoffs that give me giant NBs for at least the last twenty minutes.
However, I can chalk almost all of that up to "making a movie for people other than comic geeks". Things have to appeal to an audience of... well, everyone. If a pure comic movie was made, nobody but us geeks would see it and that would mean a lot of lost money for studios. I get that. Reynolds a an a-list actor who brings with him an audience who may not have seen Lantern otherwise. I myself would go gay for him in a heartbeat (if not for that damn restraining order). When ya get Reynolds, ya get his cornball acting. When you throw him into a love interest story, you have to take cheese and campy dialogue with it. I get that. I may not have liked it, but it's how you take Green Lantern the comic and make it into Green Lantern the movie.
My Good: Ryan Reynolds. Did I mention my profound love for him? Even through his cheesy, Reynolds still pulled off Hal. Great job of mixing the two together (as Adam and Ken both already said). And Peter Sarsgaard was brilliant was Hammond.
And I may be pissing a few people off but I loved the different take of Parallax. I don't want to ruin it for anyone who may not have seen it yet but the new way they tied him to the Green Lantern Corps was very impressive and it worked perfectly for putting Hammond into place. Very smart writing.
I would absolutely recommend you see this movie if you haven't yet. While I did leave a little disappointed (I had some amazingly high hopes), I was still very impressed. Not as good as X-Men or Thor but the critics can suck one cuz this movie was damn cool.