The drive wasn't really that bad. We paid for a GPS audio guide (Shaka Guide) and stopped at many of the suggested sites. I had 3 kids (10, 13, 15) with us and nobody complained about the length. Though my 13yo gets car sick and vomited after about the first 90 min of twists and turns. I thought it was a cool. Lots of nice waterfalls and other nice scenery.
You do have to have reservations for the black sands beach, and that was the coolest part. We left around 9AM and got to the beach around noon. Our reservation was for 1:30PM, but they let us in early because it wasn't that busy.