We stayed here for 30 days. There are some really good things about this place and some not so great things. Unfortunately the not great things would probably keep me from staying here again.
Good things:
It's probably in the best location you can get in Madrid. There are literally 2 very good grocery stores in the bottom of the building, plus maybe a dozen restaurants, cafes, bakeries, gelatarias, pizzerias, etc... right outside. In a 1-minute walk, you could eat something different everyday for several weeks, it's insane. There might seriously be 500 restaurants within a 5 minute walk radius, it felt like they never end. Even though there is a metro station at the front door, you don't need it because you can walk everywhere. We only took the metro once, didnt need to any other time. The main shopping districts are a 5 minute walk away, the palace is less than 10 minutes, the big park and big train station is 10 minutes. The apartment is REALLY QUIET. Like almost unbelievably quiet, considering that the square oustide is pretty loud at all hours. The Wi-Fi is very good, something like 500mbps down. It has all the utensils you need for cooking at home, it was well equipped. There is a washer and a clothes drying rack out a window. There are 2 elevators, there's usually a building manager/doorman who sits inside at a desk during the day and is friendly, and the interior of the building/other residents were very quiet and seemed pleasant.
The bad things and why I don't know if I could stay here again:
Number one big thing is the air conditioning... There is an A/C unit in the living and it does blow somewhat cold air. But that air doesnt make it to the bedroom, no matter if you leave it on for hours. When we stayed in June, we couldnt get the bedroom below 28-30 degrees celsius (82-85 F) which is just too hot to sleep in. There is one fan which helps, but it's not enough for two people. So we ended up sleeping on the living room couch (there is a pull out bed in the couch) most of the time. The apartment is also just a little rundown. It could use a good run through to fix and replace all of the old stuff. Most of it was small, like the utensil drawer was broken, the kitchen lightswitch was broken, the clothes hanging rack outside was a little broken, doors would squeak, the refrigerator sounded like it was dying, the toilet leaks and is loud, the pillows were terrible and we went out and bought our own, the bed was uncomfortable and felt really cheap, etc... Those kind of small things add up to an overall experience that it is a worn out apartment. Finally, the front door absolutely sucks. Even after using it maybe 100 times over 30 days, it was still hard to open from the outside. The owners mention this in their check-in documentation. It's not possible to open it with one hand, and it gets stuck a lot so you have to struggle with it. It just needs to be replaced.
Those negatives, along with the nightly price of this place, would make me look elsewhere. If you're only in Madrid for a week or less, then just stay in a hotel. If you're there for several weeks and need something bigger than a hotel, I would look at another apartment unless the convenient location is really an important factor for you.