Let me give the fairest review possible:
The Good: It's located in a great area next to many restaurants etc. the pictures of the apartment were accurate and it contains everything necessary to cook etc. The place is tastefully decorated. It was priced below other units in the area.
The Bad: Check-in was awful. The owner was not there and thus I had to pay a taxi to take me across town, wait for me while I pick up the keys, and return to the apartment. I was travelling alone so it's not like I could have left my luggage anywhere. So that was an additional 25 euro. The main issue though was I was there during a heatwave where it was nearly 100 F (40 C) everyday and the only was to cool off was a little fan by the window. It was so unbearable that I couldn't fall asleep until about 1-2 am in the morning when it cooled off. The bed itself was a foldout couch was wasn't the most comfortable thing to sleep on.
All-in-all you need to know that if you come to Madrid make sure you find a listing that says "air-conditioning" otherwise you will have many miserable sleepless nights there.