Diego's appartement was exactly what we were looking for, to be able to visit Madrid. It is very well located, very close to the center of Madrid, not far from a subway station. It has everything you need for a short stay, including a very good WiFi.
Close to the apartment you'll find a small Carrefour Express for buying small things you need for e.g. your breakfast, but there is also on the opposite side a Dia supermarket, which is a little bigger and offers more choice, both are about a three minutes walk away. Google map is your friend to find them.
We recommend Diego's appartement and would definitely come here again on a next trip to Madrid!