I think the main characters were pretty well cast in the dub, but some of the extras and minor characters were kinda hit-and-miss. I think it's one of those shows that's passable no matter which way you watch it: if you're the sort who usually watches dubs, the dub certainly isn't bad enough to make you switch, but if you're a hardcore sub purist, it won't change your mind either.
The anime really only has two major flaws: a relatively cheap budget meant they had to cut some corners animation-wise, and the series enRAB on a huge cliffhanger. Essentially, it's a 25-episode commercial for the manga. (-: The story and characters are great, though, so my advice is to check out the anime, and if you like it, be prepared to check out the manga to see how the story continues after the end.
And of course, I won't complain in the slightest if you should happen to check out the abridged version (linked in my sig) somewhere in there as well, but you'll probably enjoy it more if you watch the anime first.