Check-in was seamless even though we were a little late thanks to some airport hassle. The place was clean and simple. We visited during New Years, and with an outside temperature of around 15 °C at its highest, it did get a little chilly at times. The apartment did have a wall-mounted A/C which could presumably have been used for heating, but we could not get it to work. If you're traveling as a group of three, know that the upstairs bed is a fold-down sofa.
Location was absolutely perfect! Smack bang in the middle of Madrid with a convenience store one street over, and walking distance from an impossibly large number of top notch bars and restaurants! It's also close to Retiro Park, which is fantastic for a walk or a picnic the day after a night out in Malasaña.
The host was very helpful in answering any questions. I highly recommend this place.