This AirBnb is absolutely wonderful. The location really is in the centre of Madrid, in the very cool Malasaña neighbourhood. There are dozens of amazing tapas bars nearby (also check out the incredible La Romana ice cream shop).
The apartment is walking distance from almost everywhere - Gran Via, Plaza de España, Plaza del Sol, and the very cute La Latina area is a little bit further, but still walkable.
The interior is wonderful - everything you could need is in the apartment, including a washing machine, dishwasher and oven. It's also super clean and very modern. It's not a huge apartment, but the space is used very well, so it actually feels a lot bigger.
The balcony is a great place to catch the sun and soak up the atmosphere from the street below.
The only downside is that this is one of Madrid's main shopping streets. So although it's quiet in the daytimes because no cars are allowed in the road, at night, it can get noisy when people start leaving the bars to go home. I would definitely come back - but earplugs will help if you're a light sleeper.