I attended ECELA's Cusco school for six weeks in 2013. It was an amazing experience! When I arrived, I was the only student and the woman who owned the apartments I was staying at invited me to spend time with her family while I was alone. Soon after, though, a large group of medical Spanish students arrived and we had a lot of fun together.
The school is in a reasonable neighborhood and the instructors were friendly and knowledgeable. I took advanced classes and the dialect was rather straightforward; the accent there is not strong. Each week, the school organizes fun activities you can choose to participate in, including trips to other parts of the country. Meanwhile, I had plenty of time to explore the rest of the area, including Machu Picchu! I also visited many museums, went to an endangered animal sanctuary, went on horseback to some beautiful ruins, and went paragliding (with paragliding being on my own, not through ECELA).
I was not prepared for the Peruvian weather in May. Homes there are not heated, nor did our shower's water heater work well at all. I was very cold the entire time. However, the apartment was otherwise comfortable.