For the most part, I can't stand the voice acting on American dubs. It's not exclusive to dubs, either: American animation in general seems to have these awful voices that drive me crazy, especially as they seem to rely more on "character" voices rather than more natural-sounding voices.
However, there are a few dubs of Japanese shows that are decent. The one for the original Gundam was reasonable and Karekano's wasn't terrible, either. But for the most part, I like listening to other languages, so I stick to subs unless they're difficult to find.