Ok, I've only ridden up it on a bike but
I did drive down it in May after a horse ride.
While driving, I was sizing it up for bike descent it and I think the difficultly would be mainly in maintaining mental focus for on hour and a half or more that it would take to get all the way down. My impression is that the turns in the top portion aren't banked very much so you wouldn't want to go into a turn too hot. However, the road surface is uniformly very good which is something I wish I could say about my local roads.
I'd hesitate about starting a descent "cold" though. Maybe your time is limited, but do you think you can start riding up to the top starting at about the 8,000 foot level to get your mind and muscles warmed up? (You also get to enjoy some views this way.)
When I rode up Haleakala 10 years ago I did it on a rental bike--which was of marginal quality. One reason I decided not to ride down is that I wasn't impressed with the brakes and I also notice too late its pads looked glazed over. Personally I'd feel a lot more comfortable on my own bike for something like this, but then again, if that's all your going to do biking wise in Maui, it sounds like a hassle shipping your ride for that--so if you go the rental route, at least carefully inspect the brake pads (or simply have new ones put on) before leaving the shop.
On final note, while I can hardly state on an expert on this, I did notice driving down last Spring that the traffic about 5pm on a weekday was very light. I'm not sure what it's like in the morning after sunrise, but I'd think it would be worth checking with a local riding group or shop about the time of day when you'd be most likely to enjoy a relatively unoccupied road. This could make a big difference in the quality of your experience.