We had a decent stay at this apartment. I know a lot of blogs suggest Malasaña as the area to be in, but this location could not be beat. Do yourself a favor and try to spend a meal at Restaurante Algarabía around the corner. The menu is a little wild, but everything was crazy good. The Quail breast is to die for.
Beds are subjective I know, but I would not consider this one to be "super comfortable" like other reviews suggest. From 1 (firm) to 10 (soft) I'd rate this a 3.5 - 4, and it's creaky getting in, out, and rolling over, which can be disturbing if you're not alone. I prefer soft, so if you prefer firm then it may be more comfortable for you.
The shower may be in the top 3 best showers I've ever had. The water pressure is out of this world and there were no issues with hot water.
The WiFi was strong throughout our week stay. No issues working or doing VOiP calls.
The elevator was clutch. Make sure you scan listings and reviews to make sure the unit you are looking at has an elevator. A lot of these listings that are top floor do not, and if you have a lot of luggage it could be a problem for you.
There's one wall unit AC in the living room and a ceiling fan in the bedroom. It did the trick for the most part in Aug, but there were times when I woke up quite hot and had to crank the AC down lower than I usually would.
As far as the place looking 'bigger than the photos', as some other reviews stated, I would disagree. I think there is exactly as much room in the apartment as the photos suggest. It's a good stay for up to 2 people, and was well stocked (says my wife).
Note that you will need to stop at a city storage depot to get the keys before arriving at the unit. This was a first for me, and a little jarring, but it was actually quite easy to deal with in reality and the locker system is pretty impressive.
One of the two couches is as creaky as the bed, and if the unit were more expensive I may have been inclined to give this 4 stars due to the furniture, but given the price, location, security, elevator, WiFi, and shower, this is a 5 star stay for 1 to 2 people.