Overall:
if you're a couple looking for a place with a convenient location and don't mind a kind of "quickly put together kind of place" (the pictures are nicer than reality) then I'd recommend it. Be aware that the building is old (one can see it as having its own European charm) and you'll need to carry your bags two floors up.
Pros:
(1) Location, location, location. Metro nearby (Madrid is well connected city and most typical touristic attractions are no more than 20-30 min walking distance). There is a number of places to eat nearby (this leaves to noise at night though), including small local bars with great "bocadillos de calamares" as well as a supermarket.
(2) Sergio was a responsive host and pleasant to deal with.
Cons:
(1) The place can be very hot (we were there in June, with an avg temperature of 30C). The bedroom has a ceiling fan that is a life saver, but if one sleeps in the sofa bed, then he/she should be ready for the warmth (even sleeping with the "terrace" doors open didn't help much.
(2) The sofa bed was rather uncomfortable to sleep in.
(3) The neighbourhood is noisy at night (often very noisy). Bring ear plugs if that is problem for you. This gets amplified by the fact that we need to leave the windows open due to the high temperatures.
(4) The WiFi was rather slow (e.g., when 3 people wanted to check/post on FB -- that should be easy to fix though).