I am going to go ahead and say that I prefer to watch all my anime subbed rather than dubbed. Although, some dubs really aren't that bad, but there are some which are just plain awful.

One example I will use as a good dub is Death Note. Sure there are a few parts where it is a bit cringe worthy, but overall it is one of the best dubs out there. The voices for Light and L are some of the best dubs in anime. They left all the emotion, suspense of the story etc in with the voices. That is a good dub.

As for bad dubs, one dub springs to mind immediately: Lucky Star.
For those who have watched LS before, to watch it in Japanese makes it funnier, cuter, just all round more fun to watch. The English dub is boring, not funny (although I have to say, the 'Tomotei/Bitchin scene is funny), they get the names wrong (for example, they don't say 'Kagami', but 'Kar-gami'). Overall, I cannot stand watching LS in English.

Like I said there are good dubs, but many of them are few and far between for me. The thing that really bothers me are getting names wrong, or saying them weirdly, in the dub.