Haleakala downhill
I rode both up and down Haleakala a few years back. While going up was a challenge, the downhill was a little sketchy for me. I've done a bit of road riding, raced a little cyclocross, but nothing too crazy. I think as mentioned earlier, you really have to pay attention on the top third, especially coming around the switch-backs. I came into those corners pretty hot a couple of times and really had to lay on the brakes. I had my own bike and so was pretty confident in it. At one point a car in front of me slammed on his brakes for some animal in the road and I had to do a controlled skid. So, you definitely need to pay attention to cars and switchback corners - your speed can really get away from you at a constant 5-8% downhill grade. I also descended through the fog/clouds at one point so visibility wasn't the best for that portion.
I wouldn't hesitate to do it again if I had the opportunity, but would make a more 'controlled' descent than my first attempt.