[Edit - damn I always reply to the wrong message on these threads. This is supposed to be in reply to the questions about riding down Skyline, not about the road race]
I just thought I'd bump this as I found the info on this site helpful. I did essentially what mdavenpo2000 did. I just went last week. I got a full on downhill rig from Crater Cycles. That's just what he gave me, not sure if there was anything else, but since I was renting and never rode a downhill rig before I thought it would be fun. His hand drawn maps can be found on the Crater Cycles website as well.
His recommended route is Skyline to Mamane to Waipoli Rd. down to a park at around 3500ft. The local riders have cut some trail in between the S's along Waipoli Rd that are kind of fun. I would say the ride is well worth doing, though the trails themselves are certainly nothing better than what you can find all over Colorado where I live. Starting up above the clouds at the top of Haleakala with views of the Big Island were awesome though and it was fun to descend into the clouds and ultimately a cool way to explore a side of the island. Following this route was super easy. The only thing I think you could mess up would be missing the Mamane turnoff if you were too busy flying down the road or looking at the views. However, it's very well labeled and comes just after a slight uphill - it's hard to miss.
For any competent mountain biker these trails are no big deal and the full face + pads + full suspension downhill bike I was given with the rental was a bit overkill, though you could certainly utilize the downhill bike to haul ass if you wanted to. The ride took me about 2 hrs with a lot of screwing around and taking pictures and such. It could have easily been done in an hour. A beginner might find the loose rock on Skyline and single track on Mamane a bit unnerving.
After you finish the Mamane trail there are some trails off to your left. The owner at Crater Cycles warned me not to go down there and told me some story about guys walking their bikes for 8 hours. He also warned against taking the Waikoa loop which you will see further down the Waipoli Rd saying it was really dusty and a super long climb. I'm not entirely convinced either of these warnings is valid, but since the bike I had was a real bear to pedal uphill, I skipped them. I think if I were to do this again I would try to rent an all mountain or x-country bike and at least explore the Waikoa loop if not some of the other trail around there.
Btw, the road ride up looked cool. It was long but there were not any obscenely steep sections. Just doing the road ride down looked lame. I can't believe they have "tours" down the road. I mean come on, it's a f-ckin road, why do you need a tour guide?
In conclusion, this downhill ride is well worth doing and I'm glad I did it, but if you were to go to Maui and skip it, I don't think that would be a big deal either as there are lots of fun things to explore on the island.