Before I start: I will try to make this post as unbiased as possible, despite my dislike for the dub. If I appear biased, please take into account that I'm trying my best not to be, and if I really am being biased, please point it out to me.
Okay, here we go. I think dubs detract from the overall experience of the original, no matter how good or bad they are. Every language, not just Japanese, be it German or Russian or whatever, has its own special meaning and feel to it that you just can't convey by speaking it once translated to a different language. This is why I don't watch dubs for anything, whether it be a foreign film or anime, unless they're horrendously bad in which I proceed to make fun of them.
On the subject of the One Piece dub, it's one of those horrendously bad ones. I've seen a few episodes here and there. Here's what I think of it in detail:
Bringing more fans to the series:
Looks good on the outside. The truth is these fans don't know what One Piece is really like. To them Laboon is an iceburg and Gum Gum Blast is one of Luffy's signature moves. It creates fans of something almost entirely different, who, in turn, probably will end up like those exposed to the DBZ dub (omg y are u calling hercule mr satan) and will think oddly of those exposed to the sub. However, in some cases, some may develop an interest to see the original Japanese version (like myself with early anime that I watched,) creating more fans of the original One Piece. Which is a good thing.
Bringing the One Piece franchise to America:
Once again appears nice on the outside, but in reality isn't that great. The games brought here are redone to be like the dub, which in turn makes them about that much less enjoyable. The voices are just as bad, except for maybe Nami's, the attack names are changed, and Zoro is still Zolo. Importing is still a better option. Although, all of this is great for those who can't get their hands on an import.
Giving One Piece a bad name in America:
Uh-oh. This is where it gets bad. I tell my friends, "You should watch One Piece, it's awesome." "It's a little too goofy for me," they say. Why do they say this? Because of supersoaker guns, rubber knives, Helmeppo's.. hammer contraption.. thing, poison suction cups, Gum Gum Blast, David Moo.. you get the point. One Piece has been so altered from what it was that some people can't understand why anyone would like something so ridiculously.. ridiculous. Of course, some people still like it, but refer to my first section of this.. opinion thing.. for details of my opinion on that.
The dub also allows people with bad/horrible internet connections to have access to the anime, which is a good thing, despite it being severely messed with. They are least getting some exposure to it, and even people with bad internet connections can read the manga (Null's scanlations are the best choice in my opinion) if they so choose, so they have the option of knowing what's really going on while having the chance to see (some of it) animated. To these people who have bad internet connections and cannot get KF's or Gerusama's eps, and are restricted to the dub: I highly suggest you read the manga so you know what's really going on.
That's my input, feel free to criticize it/agree with it/disagree with it as much as you want. Sorry for the huge post!