We (my wife and I) stayed in Mendoza for 5 nights at the end of January 2011. We went on an organised wine tour which was excellent. After the tour was finished we talked to our driver Miguel, who speaks very good English, and discussed the possibility of other days out. I think his email is firstname.lastname@example.org .
Together with Miguel we planned two day trips, one to Aconcagua and the other to the Uco Valley. Miguel picked us up from the hotel at 9am and we were off on two action packed days out. We saw all the things we wanted plus much more besides, Miguel suggested places to eat which we really enjoyed. We travelled on tarmac roads and dirt roads all in the comfort of Miguels large air-conditioned car.
Miguel was full of information about where to eat, visit and buy gifts. All the sorts of things that make a stay easier. This is not a cheap option (about 1000 pesos/day all in), it would have been cheaper to have hired a car but we would not have benefited from Miguels company and local knowledge. If you want your money to stay in local hands rather than a multi national car hire company, you don't fancy driving in Mendoza, you are not sure of the best routes or you just fancy being driven about for the day then please use Miguel.