We stayed here for 3 nights in September and had a really lovely stay. The flat is comfortable and spacious for two people, and the location is ideal (15 mins walk to Retiro Park).
The light in the kitchen was not working properly and constantly flashed on and off. I resolved this by putting one of the other lamps in the kitchen but it was still quite dark.
However, there was wi-fi, which was not listed on the website and a nice surprise.
Unfortunately, on our last day, we were told by Ramon that we'd be able to use lockers in Central Madrid (as we couldn't leave them there after check out), but after walking 20 minutes to the location we found it closed. I'd recommend always checking online, just in case.
There is also no plug in the wall for the TV in front of the bedroom, so this may be an issue if you were planning on watching TV in bed. Other than that, all was in place as expected.
The above are all the little issues we had but honestly, other than that, it was a really lovely stay and I would definitely recommend it.