My wife and I have enjoyed staying at this apartment for a week very much. Aside from two important things that I will point out in the end, everything was perfect. Starting from the check in everything was fabulous. The hosts met us near the apartment, showed us everything, left for us some fruit, water, coffee, and wine, sent us a long list of recommendations on what to see around the area.
The apartment itself was squeaky clean, had everything that you may ever need, right down to a sewing kit, not mentioning regular stuff like towels, TP, toiletries, etc. The bed is amazing, the mattress seems to be new, we slept like babies.
Equipment TLDR: there's AC, fridge, freezer, washer, dishwasher, iron, ironing board, coffee maker, sewing kit, towel warmer.
The two things that we found odd about this place is that:
1 - it has no windows, it's a first floor apartment, the only "window" is the door out to the street; we didn't find this a problem at all because we only got back to the place to sleep in the evenings. Sleeping in the dark, though, is kinda strange, so we tried to leave the light on.
2 - it has thin walls, so you can hear stuff that's happening in the building, and the others will also hear you, but it wasn't a problem for us either- nights are very quiet, the street is silent even though it's Malasaña. It seems like everyone kinda walks around it even during the busiest hours of the week.
So, overall, recommended! If the two things I mentioned are not a problem for you either, go for it, you won't regret it. Everything that's to see in Madrid is very close by, about 5-10 minutes walking, even the train station is like 25 minutes.