There's screw ups in everything. A lot of dubs suck, some are good. A lot of subs are good, a lot are bad. If you're buying an dvd of a licensed anime, make sure you get the official release. #1 It's legal and #2 the subs won't be as bad I'm sure.
Sometimes I'll buy DVD's of anime that aren't licensed, and it's a serious gamble. The subs on my Mahoujin Guru Guru DVD's or Kare Kano or Steel Angel Kurumi (when they weren't licensed) were terrible. But I do have good ones, like Neko no Ongaishi or some LA movies such as Returner were awesome subs.