We stayed 3 nights at David's apartment and everything was great.
David helped us from the beginning to find the address and was very helpful and full of good advices about Madrid.
The apartment is clean, very cozy and calm even if it's just behind Madrid's biggest street. The location is also very convenient, in the very cool neighbourhood of Malasana and close from everywhere. We were able to do everything by foot.
I would recommend the apartment with no hesitation