I am a doctorate student at the University of Michigan, and I just spent two months in Ethiopia doing research on gelada monkeys in the Simien Mountains and Guassa Plateau. I just wanted to say that I recommend Yama Tours Ethiopia to everyone reading this. This was a fair, nice, reliable, great english-speaking agency to work with: http://www.yamatoursethiopia.com/.
I mostly dealt with Mr. Tariku W/aregay at the Tour agency and he helped me book my plane to Gondar and back to Addis, as well as hired a car to take me to the Guassa Plateau and back. From what I've heard from other researchers who have worked in Ethiopia for a year or more, his prices were right where they should be. Also, he can arrange transportation with English-speaking drivers if that is what you want. Plus, he was fast at responding to emails. I was also a female traveling alone and I never for one moment felt uncomfortable with any of Yama Tours employees.
The Yama Tours Agency deal with everyday travelers like myself, in addition to well-respected diplomats. Whether you need help finding a hotel, traveling safely to key destinations in Addis, or trekking to some of the historic spots in Ethiopia - I recommend Yama Tours hands down. I go back to Ethiopia for a 4-month period soon and I have no doubt that I will work with this company again.