I remember seeing the movie when it came out, and I thought that it was, at worst, entertaining. I know that people didn't really care for Ryan Reynolds, but I didn't have a problem with him. I did think that Hal was a bit of an a-you know what, but he became better as the movie went on. His relationship with Sinestro was cool as well. Overall, I think that the movie is not as bad as it's reputation. I certainly don't think that it sunk the DC movie verse. Come to think of it, I wouldn't mind seeing a Green Lantern TV show, because DC seems to do TV better than movies.
...maybe not as much as Marvels movies but it was decent. Add in anyone who wasn't a comic book fan who I made watch it...actually liked it.
So its got much worse rep than it deserves, though it was clearly flawed in areas.