I'm assuming you mean the route that runs across the Island down to the coast past the Tedeschi Winery before turning up towards Hana.

I was on the road back in October 04. It was a pleasant drive almost all the way around with breathtaking scenery on the backside of Haleakala, and I actually thought the famous unpaved parts were paved over - until I reached them There are two stretches of road that were abit uncomfortable driving on - one longer stretch of pressed gravel was OK, but bumpy with quite a few rental car unfriendly pebbles flying about. Another shorter stretch had larger sized potholes and depressions that you had to navigate around and this part was the last choppy portion of the road until the paving re-appeared a few miles south of the pools.

I think a mountain bike should be OK for this road, but be aware that passing cars and vans might throw up some pebbly debris as they pass you, so dress and protect accordingly. If there are rains prior to your arrival, you may want to rethink the trip, as the unpaved parts could become muddy.