Most all rental car companies on Maui will allow you to drive to Hana. Driving the backside of Haleakala, past Oheo Gulch, goes against most Maui rental car agreements. ROAD is open: The best parts of driving to Hana are between Paia and Seven Sacred Pools, so it shouldn't be an issue.
4WD is not necessary in making the drive to Hana. If you plan on going off of the Highway, you may need one due to muddy unpaved roads, but you're not likely to do this.
Though the winding, cliffy road to Hana is more dangerous than most, the reasons for its infamy come from its history. A couple decades ago, the Highway was poorly paved and unsafe. It had potholes, weak old bridges and missing guardrails. With the inception of heightened tourism, the Road to Hana is well paved and safer than ever. Dangers still exist though. Being subject to constant rainfall, the road is wet in many areas along the drive. This combined with high cliffs, low-visibility hairpin turns, narrow roads, speeding locals, and gawking tourists can make this road treacherous. Make certain that the driver is watching the road. Switching off is the best way to ensure a safe and pleasurable drive.
It really depends on which one you get. The R2H "Experiencing The Road to Hana and Beyond" is worth buying because it offers much more than an audio guide through Hana. With this CD, descriptive folding map, and driving tips, you get the full experience. If you want a journey you'll never forget, memories filled with romance and adventure, it's worth getting this CD.
With hundreds of cars driving this road everyday, it's difficult to say. On one hand, it's a shame to leave with the pack. On the other, the road is long and takes time to drive. Coming back in the dark is pointless. If it's dark when you drive back, you'll miss the reason for driving it at all. It's best to leave early, even if it means driving with everyone else. Make sure to stop and enjoy the view when you get backed up. Pull over and take advantage of your senses.
You can also stay overnight in Hana, which is highly suggested. Travaasa is one of the best resorts Maui has to offer and it's right in Hana Town on the water.
If you don't want to spend the whole day driving, consider driving to Twin Falls, which are the closest and most easily accessed falls. There are some great hikes here, and the smoothies, and fresh fruit at the stand make it worth stopping at. Wailele Farm owns the property here and leave it open to the public.
Yes, you can fly instead. Up until recently, flying to Hana from Kahului was rare and expensive. Mokulele Airlines has recently opened up 2 daily round-trip flights to Hana at a great price. This is good news for those who get car sick, or don't have the time for driving (ex. a short trip to Hana.)
If you're looking for a great aerial tour of Hana and the sites along The Road to Hana, Blue Hawaiian Helicopters offers tours of Hana/Haleakala and Complete Island Tours. This is an incredible, thrilling way to see the road to Hana from a really unique perspective. You'll see sights that are impossible to see from the road.
Visit Blue Hawaiian Helicopters or call 1-800-745-2583 for tour and booking info.