I really wanted to do a ride up Haleakala last fall, but I ended up only having time for Oahu and the Big Island. I considered riding up Mauna Kea from Hilo, but didn't have time.
With some of the responses here, maybe its a good thing I didn't get to Maui. I thought the traffic on Oahu was terrible, and the Big Island was only slightly better. Maui being even worse is almost hard to believe. I guess there are too many tourists (I didn't help there obviously). I guess I thought because of the popularity of the downhill ride that up Haleakala would be good too, but if the road and traffic is as bad as has been said, that's too bad.
-Joel