My husband and I did a trek in the Simien mountains - one of the highlights of our trip to Ethiopia.  Afera, our guide, was expert and knowledgeable about the geology, flora, fauna and people of the area.  (He is also an interesting, friendly and fun person whom we enjoyed spending the days with!)  The logistics of the trek were very well organized  with respect to food, gear, animals, etc.  We highly recommend Afera.