Are you a purist otaku who thinks that subs are unconditionally better than dubs? If you are, then you might not want to read any further, because I'm about to wreck your shit. Consider how much the first episode of the Saiyuki dub owns the sub:
For the record, "Nyoi-bou" doesn't translate to craftily dubbed "OH SHIT", but does that really matter? You never actually need to know what Nyoi-bou means, so why complicate things when you can just insert an expletive?
Now let's take a look at Hakkai:
Some shows are meant to be watched dubbed. Since Saiyuki is full of one-liners, the flow and phrasing of the dub is siginificantly more important than the "true" overall meaning that you'd get from the subs. You should also watch the dub if the sub gave a girly voice to the male lead, especially if it's a fighting series:
How can you be intimidated by a guy who sounds like a little girl?
I'm not saying that all dubs are good. If you have to make an uninformed choice between a sub and
a dub, then it's probably a safer bet to choose the sub, but if you have to opportunity to see both
before choosing, take it.
UPDATE: I found these remarkably well made clips that capture the sentiment of this article perfectly:
1554 elitist bastards still refuse to watch dubs