ok, let me be the first to say, "its shit!" - from my experience back in January.
and calling it a road is indeed being magnanimous.
But otherwise, like you say, its the main road.
So, don't leave Nairobi without your visa for Ethiopia. Being told to go back to Nairobi to get it would leave you crying.
Its just a deeply rutted, rough, hard, rocky, pista of 550 kms or so, you can do in two days. Other than it never quits, never lets up, just pounds you for all 550 kms, its really not that bad. Unless of course, you hit it during rain season! Suerte!
BTW: the Long Way Down boys - this is where they blew out 4 shocks. So if you don't have support behind you carrying extra rear shocks, then you need to try to figure out how to save the one you got!
Chiang Mai, Thailand