I am planning to drive the Turkana route from Ethiopia to Kenya. I saw postings on the Kenyan part. But what about Addis to Ileret? What is the best route in terms of road conditions?

Well, on the final stretch to Iliret there is only one route: Turmi - Omo - Iliret. The way to Turmi isn't really much of an issue (except that they sometimes run out of petrol). If you are prepared for Turkana and have the necessary equipment and skills, road conditions on the way to Omo are more or less irrelevant (except for traffic). Instead I would go by what you intend to see, e.g. stopping by Jinka to visit Mursi tribes will probably appeal. Arba minch and the lake are also nice, and a more or less safe bet to get fuel.

As far as the route from Addis to Arba or Jinka, it would be best if you ask for recent conditions at Wim's Holland House in Addis, which would be my recommendation for a place to stay. Most likely the recommendation would be Addis - Butajira - Hosaena (not via 51 but minor road). But ask Wim when you are there, he'll fill you in on the latest details.

Edit: I just saw your question in the Turkana thread, asking for the fastest route from Addis to Iliret. If speed is a priority for you, I would not recommend to do Turkana, but take the Moyale road instead. I don't know your skills and which kind of backup you will have (how many vehicles?), but Turkana is remote and I found taking my time is important to prevent disasters. After all, it is one of the places on this planet where help is far if things go wrong ...

Thanks Sirakor. I prefer the Turkana-route. Will probably drive North-Horr to Marsabit. I am not really looking for the fastest route to Nairobi, but the fastest route through Omorate to Nairobi!
By the way: Wim died some time ago.