I'm a much bigger dub supporter than a lot of other people I know (and a lot of other people in this thread, it seems). Don't get me wrong; I love watching anime in its original language, too, and there are plenty of shows I think have superior voice work in the Japanese than in English (the above example of GTO is a good one). HOWEVER, I think there are a lot of quality dubs that can rival their Japanese counterparts as far as the actual acting/emoting is concerned, particularly with shows that were dubbed in the late 90s/early 2000s. Cowboy Bebop, Trigun, FLCL, and Hellsing (and the Hellsing OVAs, for that matter) are prime examples of this (the cast of FLCL's English dub is so close to its Japanese equivalent it's just downright eerie in my opinion). I think the Death Note dub was a good example of something more recent that came out well, too (and, to a lesser extent, Code Geass and FMA has pretty decent dubs as well, at least in my opinion).

As others have mentioned in the thread, I think people tend to hate on dubs because they understand the language and can be more critical of an English performance than they can a Japanese performance. Even with several years of Japanese under my belt, I wouldn't feel properly qualified to critique the nuances of any given performance in an anime.

Of course, there's a very valid argument to be made in the "purist" vein. Namely, if we can treat international films and operas as "works of art" that don't require (vocal) English translations, than anime shouldn't either, and I'm fine with that argument.

I think overall I'm pretty down the middle with how much I watch subbed and how much I watch dubbed, but I definitely wouldn't rule out a show's dub on the point-blank assumption that the Japanese would automatically be better. All I can say is give them a chance; they're not all disappointing!