I haven't seen the film in years (or remember it quite well, either) so I can't really give much of an opinion on it, though I doubt it's as bad as they say.
Supposedly it was kind of an "Avatar that flopped," so to speak. Huge budget, and tons of hype, but it didn't live up to it's claims (according to critics anyway), and apparently people were "getting sick of Kevin Costner appearing in so many films in recent memory". Then there is always the group that felt the whole "Global Warming plot" was too "preachy/"non-scientifically based" for it to be considered a "good movie." Also the special effects didn't turn out so "astoundingly great" (which was originally a selling point), mainly due to the fact that a hurricane ripped apart a number of sets during filming. Or so I've heard anyways.