There is one spot (a few hundred yards long) where the road gets quite narrow. But there are several turn off places to get by. The road widens and gets a lot better after this. I've driven it many times even in a large vehicle with no problem.
But if you tend to be REALLY nervous when you drive, go as far as you feel comfortable and turn around.
Stop along the road (lots of parking spots) to see the sights, including a really nice blow hole. Check out "Maui Revealed" for lots of places to enjoy.
It's best to go in the morning. It will take several hours, so you may want to have lunch in Kahului.
HOT INSIDE TIP: Have lunch at the Dunes golf club. The public is invited; the scenery is nice; and the food is good.