Personally I do it both ways whenever possible. Its hard to come by good english voice actors, and so alot of dubs I've seen are about 40-60% the same quality as the original voice cast. Of course alot of that has to do with budget.
There are however some really good english dubs of shows. Cowboy Bebop, Bastard!!, Eureka 7, Ghost in The Shell, .hack, and some others.
I still like subs though. Mainly because, sometimes there are expressions, words, jokes, references, or jargon that only the Japanese would get unless some description is given with it. So when you watch the Japanese version with subs, alot of groups with explain what is going on, which is turn makes that scene more enjoyable.
There are also some shows that are plain horrible in dubs. Initial D, is probably by far, the worst one I've come across. A decent show by itself, but when I watched the english version it sucked. They changed the original Eurobeat soundtrack to some weird indie rap & rock stuff. And the voice actors were plain bad.
The same can be applied to subs. Sometimes the quality is bad due to age (Boys Over Flowers for example), or the sub group isn't very good.
Still, I think people show watch both english dubs and japanese w/ subs to get the full feel of it.
