I just returned from a week at Nuevo Hotel Callao and I had a great experience. My room was spotlessly cleaned each day, the coffee/juice/croissant breakfasts were served in a lovely room by the friendly cafetera, Paolina, and the staff went out of their way to ensure that we had what we needed. The hotel is very safe and in a great location for getting around Buenos Aires. Staff doesn't speak much English, so if you don't speak Spanish, bring a dictionary. Also, there is street noise in the outer rooms, so ask for an room on the inner part of the building.