Not extremely terrible, just horribly generic, formulaic.
If you are looking for just another superhero film, Green Lantern is great.
So, billions of years ago, there existed a group of intergalactic space cops called the Green Lantern Corp. Their powers come from a magical green ring which uses the power of "Will" to wish anything into existence. The power of "Will" directly contrasts the "Fear" which is yellow in colour. Billions of years ago, the galactic council rulers fought against the evil Fear-using Parallax Smoke Monster. Parallax was all about destroying worlds. When Parallax was defeated, he was imprisoned in an asteroid in the 'lost' sector. Well, six months ago a space ship crashed on the asteroid and accidentally released Parallax. Oops.
Well, Ryan Reynolds eventually gets one of these rings and the origin story continues.
I want to give special props to the bald scientist played by Peter Sarsgaard. He has the most interesting plot in the story… at least before he became horribly ugly. Something that didn't jive with me is the casting of his character's father. IMDB states that Peter is 40 while his movie dad is 52.
There's a bunch of plot weirdness in the film which can be overlooked if you don't attempt to think about it. That's pretty standard fare in genre films though. For example, Ryan Reynolds got teleported to a dock/beach and discovers an alien/ alien ship and calls his best friend to pick him up. His best friend knows about the alien, a magical ring, and a lantern. In the next scene Ryan Reynolds is all alone in his house playing with the ring. Why would the best friend, being a scientist, not want to investigate the ring with Ryan Reynolds? Didn't make sense.
Another example is the prison of Parallax. So this guy can destroy whole worlds and actually wants to dominate and rule the entire universe. He was defeated… but not killed. I wouldn't want to have a threat like that alive, so there better be a reason for not killing it. Anway, the galactic council decided to put this galactic evil on a remote asteroid with security so lame that three dudes in space suits accidentally freed it by stepping on the surface of the asteroid. The deaths of billions are on you, immortal guardians of the universe. Oh, and I love the intro scene where the three aliens were calling for help in an alien language (subtitled), but when Parallax starts to escape, he was speaking English. I understand that the Green Lantern Rings have translator capabilities. That's cool. Why was Parallax speaking in English? They could have avoided the problem entirely if the three aliens were speaking English at the start too. Eh.
Green Lantern isn't bad. There are a couple of laughs in the film. There's some clunky dialogue. There's some neat visuals. In the end, it isn't as deep as 'The Dark Knight" nor as entertaining as "The Green Hornet". Just some mediocre superhero film.