It’s all about the quality of the voice acting. Some dubs are horrific, and really take away from the overall quality. Others are good enough to go unnoticed, as long as you aren’t expecting the original.
The dubbed Cowboy Bebop is great. Trigun didn’t bother me, not did the adult swim release of Samurai Champloo… which was remixed or something. Can’t watch the dvd release now, because I loved that remix action.
I liked the Ghost In The Shell dub for the series, not so much for the movie.
My girlfriend has trouble with reading subs without her glasses (which she hates wearing, even though she looks cute in 'em) so if I can convince her watch any anime at all, it has to be dubbed.