We stayed at Pablo y Marisa's place twice (at the beginning and end of our trip) and loved it. The location is great in Plaza de la Merced and the area is very walkable. They were easy to communicate with and everything went smoothly.
A few things to note: the flat is up five flights of stairs, the WiFi can be spotty if you have a big group, and it can be loud at night if you have the windows open. They just installed air conditioning, though, so that's now an option.