Positives: Easy checkin. Great location. Low price. Good communication. Free coffee (bring your own sugar) and muffins. Lots of good nearby restaurants. Towels and sheets provided. On a low floor in the building.
Negatives: thin plywood walls. Shelves inexplicably located at 7+ feet from the floor, no useful shelves near the bed. No light control near the bed. The bed is actually about 4 feet from the floor so you'll need a step ladder to get in/out of bed. There's no place to sit down in the bedroom or in the common room, not even a chair. The room fan needs to be on all night and the room window (to the kitchen) needs to be open to get some air circulation. Some mosquitos. The common shower is big enough if you're not a big person. The bed is about 170cm long, so either you sleep curled on your side or diagonally. And he offers a post-stay refund of the cleaning fee for 5 star reviews, which could lead to some skewed reviews.
It's good if you just want a very basic room in a great location in Madrid at a very low price.