I never read the comic books but I watched the movie and although it wasn't well written or smart, it was at least fun and had a lot of cool visual effects.
It was, at best a mediocre movie but everyone on internet says it was a disaster.
Overall Green Lantern was a train wreck of a film. In short, it sucked.. But "why?", I hear you ask...
The film has achieved just 5.7 on IMDB and a 26% on Rotten Tomatoes. Although the initial reception (e.g. before seeing it) was generally positive, after seeing it, people failed to recommend the film to their friends.
Pretty much the most positive professional review I could find comes from the Examiner
Now, I did not see this movie in 3D, so I cannot vouch for any difference in the moviegoer experience, but Green Lantern (2011) was mediocre at best. It seems to serve as a stepping stone into future Lantern flicks instead of a wholesome headlining franchise experience. Would I recommend this film to others? Sure, why not…it is the summer after all.
Hardly high praise...
According to Box Office Mojo, the film cost some $200M to make and has recouped (to date) less than $230M. Although this isn't an actual loss, it's considered to be a crashing disaster when you take into account the lost opportunity costs (e.g. if you'd made a better film) and the extra marketing and advertising spending that isn't factored into the production costs.
After the film's failure, Warner Bros (and the main cast and crew) cancelled their plans to turn the film in to a film series. Despite the rich mine of potential storylines available from the comic canon, the film is now basically orphaned. In 15-20 years, they may try a reboot, pretending the 2011 film never happened.
Given the comparable recent success of the X-Men film series, Superman film series, Batman film series, etc this single movie failure may have robbed the studio of a franchise worth billions of dollars.