Just got back from a 3-day trek this past weekend to Colca Canyon. I went with a company that was recommended to me by a couple of German travelers I met on a bus: Land Adventures. I thought it was well worth the $$, time and occasional huffing & puffing. Very professional, good food, English-speaking guide, fantastic views, and a condor sighting. They´re located on Santa Catalina (near the Plaza de Armas) in Arequipa. I´d give them a look: they seemed to have a lot of satisfied customers.