We always hire private guides when we travel because I am semi-disabled and can not keep up with an organized tour group. We just did three weeks in Peru and the guide we had for Machu Picchu, Cusco, and Lake Titicaca was fantastic. Not only did he know his stuff, he knew people everywhere and was able to get me in to places and sites that I otherwise would not have been able to access. The only poor experience I can think of was in Zambia where our private guide kept up a steady stream of political commentary, much of which I knew was not quite accurate from the extensive reading I had done before going. I didn't challenge him, though. Not worth the effort.