Mike's flat was a great option for us and we recommend it. We went to Madrid for 4 nights in the height of the summer and it was extremely hot that weekend. Thank god for the AC!
The check in is a bit more complicated than other places but Mike explains it very well and we had Google maps. You have to go to this small convenience store and ask for Mike's keys. The man in the shop gives them to you and you continue to the flat which is very close.
The flat is small but charming and it has everything you might need. Plus the price and location are unbeatable. For the 3 of us that went, it was just what we needed.
A thing to point out is that there isn't a lock in the bathroom but we didn't have an issue with it, just let the others in the flat know you are using it. In regards to the boiler having a specific capacity for hot water, that wasn't an issue at all for us.
Also, for the kettle to work, the kitchen light has to be on.
Sometimes it can get loud at night if a couple of people decide to talk down in the street. Unfortunately, it happened a couple of nights but otherwise it's very silent. (We also went the weekend of the Eurocup final).
Mike was also a great host. We didn't meet him but he provides a lot of information and checks in on you when necessary.
Overall, we were very happy and would go back!