I can't really understand all the negative reviews here: you can almost hear the comic book guy from the Simpsons declaring it "worst. episode. ever".
The first Futurama movie didn't quite work for me. Entertaining, but it seemed like the writers were struggling to fill the feature length, and chose to fill it with so-so material which would have been cut on an ordinary show (eg. the early "Fox executives themselves fired" joke dragged on far too long).
The second was much better, particularly the first half (the Steamboat Willie spoof, the Deathball scenes, Disco boogie).
The third one may be slightly less entertaining than the second, but still has more laughs than you'll get from just about any show still on TV!
So, ignore the naysayers. Buy it and make your own mind up.