do it all. go to black rock and swim with the giant sea turtles while you are in Lahaina its a really cool experience.
also i took the rental car all the way around
pretty much if you do something to the rental you are fucked tho so that is at your own risk. i had to do it tho cause i rode a harley around the north side so i wanted to say i have been around the entire thing. Anyhow South of Hana is pretty epic. I really enjoyed. here is a vid i put together of my last trip there....
[ame="http://www.youtube.com/watch?v=479P7b4bUHM&feature=plcp"]Black Rock Maui Fixed - YouTube[/ame]