Sergio's apartment is very comfortable and has a good feel about it. Its bright and the sleeping part (2 bedrooms) is separate from the living area. It is a typical man's flat! Very few things, which we actually liked.
The location is in a residential area of Madrid, so if you want to be in the middle of all the hustle and the museums, it is a bit on the side, but it is what we were looking for so it was perfect (the museums are a 25min walk).
The only real downside was the communication. When we arrived, we tried contacting Sergio for about 45min to get directions about where to pick up the keys etc. and he was non responsive. As we were with a 4 month old baby we started to look for a hotel when finally Sergio returned our calls. Similarly on departure, we agreed he would come to pick up the keys at 3pm and still at 3:30 he was not there and had not messaged us to let us know what was going on. In the end we had to leave the keys in the mailbox to avoid being late for our flight. These were quite stressful moments and could have been avoided.
On the other hand, he recommended some great local restaurants and was very helpful otherwise.