I stayed at Melvin's apartment together with a friend of mine. It was very central, right next to gran via. We were able to walk everywhere, no public transport needed, so that was nice.
Check-in and -out was very professional. We never met Melvin, but everything went smoothly. He seems to have some assistants, who did a good job. We were able to store our bags almost all day after check-out.
The interior of the apartment is pretty basic, and the walls are thin, so no family location. But for a short city visit, especially for young people, it's more than perfect.
Good price, very close to the more hip parts of Madrid, and has everything you need.