I've done all or part of this trip three times now in violation of the rental agreement. Thought I would share my experiences.
The first time counter-clockwise it was extremely rough the whole way and we turned around probably half way to Kipahulu. The second time was 5 years ago and again did it counter clockwise. The first 15 miles from Ulupalakua was smooth blacktop then it turned back to extremely rough. My wife did the last half of the drive to Kipahulu while I sat next to the guard rail looking down the cliffs at the ocean.
A month ago we went clockwise for the first time. I drove from Hana and since I didn't drive that section the last time, I didn't realize how tight the road was past Kipahulu. I definitely would do it clockwise or get up very early to go the other way. Some of the tour companies take small buses all the way around now. I would hate to meet one of those buses the first few miles past Kipahulu.
I tried to measure the distances between Kipahulu and Ulupalakua this time. The 30 miles was pretty evenly split, 15 miles of dirt and patch on top of patch pavement and 15 miles of nice smooth pavement. Of the 15 miles of rough road, only 3-4 miles are dirt. The dirt part was actually better than some spots of the patched pavement. The first 5-6 miles past Kipahulu is a much rougher and narrower version of the Road to Hana. It straightens up some and windens a little past that.
If the rental car is in good shape like it should be you shouldn't have any problem. The twists, curves, and road condition pretty much force you to take it slow. The transition from the windard to leeward is very interesting and there are some awesome views. I don't regret doing it.