Bridget and I had a lovely time at Hass' apartment. Though it's a walk-up apartment, the stairs really aren't that difficult to get up if you don't have a handicap. There were three different doors that required keys to access the room; we liked that sense of security.
It generally had what we didn't bring -- towels, shampoo, etc. -- though it would have been nice to have had some foreign style plug converters to use. Definitely not something we expected, but it would have been a nice touch.
The apartment is quite conveniently located, right by the palace and all the downtown tourist hot spots. Outside of those popular places, the neighborhood of La Latina was right at our feet, and truly Madrid's center is so walkable that it is really not a difficult endeavor to walk the 40 minutes to Malasana and Chueca (provided you stop for a tapa and a glass of wine every so often along the way).
The main concern we would state is that the apartment was not quite as clean as we would have liked. There was food and a drink from the guests who had previously stayed there in the fridge (not a major deal) & there were a few stains on the sheets, there were spares we ended up using though.
The air conditioner also had trouble getting revved up, but neither the cleanliness or the air conditioner issue seems to be representative of the typical stay at Hass's, based on everyone's reviews. Despite these small things, I would still recommend staying at the very lovely apartment with the nice, airy balcony.