My fiancé spent quite a bit of time cycling on Maui - he would go there quite often with his family - and I think he'd say everywhere is good. But he's a very strong and experienced cyclist, so YMMV.
He has rented bikes at the South Maui Bicycle Shop (I believe) http://www.stirflux.com/smb/ and was very satisfied with what they had.
Regarding clothes, well it depends on when you're going I guess. I'll let others who know the area better answer to you on that!