This was an incredible ride. I highly recommend it!
We bike camped and completed the first leg (Pukalani to Kipahulu State Park) on Day 1, had a very short Day 2 to Waianapanapa State Park / Black Sand Beach, and ended at the bus stop in Hai'ku to ride back to Kaanapali.
Day 1 was dry, with lots of pastures, goats, cows, and little water (bring water!). There were about 8 miles of unimproved / gravel ish roads. With the many varied patches of road covering, I pictured an antiquated truck spattering out asphalt all over. It was definitely different to maneuver than gravel but I did fine with my 28" tires, and didn't get any flats.
I definitely recommend Kipahulu to camp at! It was frustrating that bikes cost $12 / bike to camp, while entire cars are charged $20, but it was also the single most beautiful bike camping spot I've ever slept at! We had to move our tent half way through the night because I thought we might be swept away or the tent poles busted. The wind was fierce near the cliff overlooking the ocean.
Day 2 we ate at Braddah Hutts BBQ outside of Hana. YUM. The ride was relaxed but we were still tired from the long bus rides and late day of riding the day before.
At Waianapanapa State Park you camp on some very plush grass amongst ~30 other families.
Day 3 was so gorgeous, all rainstorms and sunshine and so many shades of green, plus some domesticated cats and lots of honor system fruit stands. Every turn was a hairpin turn. Tons of steep cliffs and rusted, abandoned / crashed vehicles. I guess there were 56 bridges! Drivers were extremely cautious, often driving right up against the mountain to give us ample room to pass. I think Hana Highway must be much scarier in a car.
About the route:
We took the route the opposite way people drive it (the typical journey is for people to take the "Road to Hana" from the north in the morning, and drive south, then west back toward Kahului). Since we were opposite traffic we had a very relaxed bike trip and were hardly passed by anyone from behind. Our trip was Monday - Wednesday.
The most difficult part was trying to coordinate four people with bikes on Maui's bus system, since all buses will carry strictly two bikes total, and many buses run every 1 / 1.5 / 3 hours. It was difficult to stagger, so half our party took the bus to Kula, shaving off 10 miles of climbing, but started later in the day. We went with the trick of being ridiculously early to the first bus stop in each town, to guarantee two bike spaces (and felt bad for locals who might have missed out, but locals don't seem to depend on the bus system for their bikes).