We recently went on a wonderful four-day trek in the Simien mountains - spectacular scenery and lots of wildlife, include Ethiopian wolves, walias and gelada baboons - and I want to recommend the organizer we used, Dezy.
I have done quite a bit of travelling around Ethiopia and there have always been some hitches with the tour companies I've used, but with Dezy the whole thing was extremely well-organized and I felt very relaxed and safe. He was very punctual, reliable, efficient and fun to be with. Dezy organized the whole thing - transport, camping equipment, food, cook, mule, scouts, guide, etc. - and I felt the price he charged was very reasonable. If you are planning a trip to the Simien mountains, I would thoroughly recommend using Dezy.