Edwards was the smart choice for Kerry to make. Realistically, VP candidates can do a lot more damage than they can do to help. Edwards certainly doesn't hurt Kerry.
I doubt he'll help much in the south, but the south isn't exactly a Democratic stronghold. Edwards is a good partner for Kerry in that he is young, energetic, charismatic and a bit more moderate in some of his views -- basically he shores up some of the weaknesses in Kerry.