We stayed two night's in Bibi & Gigi's nice apartment. The location is great and you are right in the middle of Madrid. You can reach basically all important locations by foot and also be at the airport in little more than 30min.
The apartment has all the necessary facilities and is well maintained (no wonder, since the hosts live there themselves). The room is right under the roof and quite cozy - especially if sunshine is falling through the window. It's perfectly fine for spending the nights while exploring the city during the days. However, if you plan to spend a considerable portion of the day inside, I guess it would be better to pick something bigger.

All in all, I can certainly recommend this location,