Pros: The location of the apartment is fantastic. We easily walked everywhere we wanted to go in the city. And while it's very centrally located, the street was quite quiet and you couldn't hear any street noise in the apartment. Plus, even though it is on the third floor, there is a reliable elevator, which can be a rare find! The host/management company supplied all the amenities we needed in the bathroom and kitchen (except, as someone else mentioned, there were no napkins/paper towels in the kitchen so we had to buy those). Everything looks very nice and the TV and wifi worked great.
Cons: It was exceptionally hot during our visit with temps around 38 C / 100 F. The AC unit in the bedroom worked great for that room, and then we were provided with a fan in the living room that did the job. However, the bathroom was really hot and the AC never would have been able to adequately cool down both the bedroom and bathroom. While we found the beds in the bedroom to be comfortable, the sofa bed was not so great. Something to keep in mind depending on number of people and sleeping arrangements. Final con is around cleanliness, as we found the floors could be cleaner, particularly in the bedroom and bathroom, and the duvets had a number of stains.
All that said, we would stay here again if we're ever back in Madrid!