I found the Airbnb to be very cozy and comfortable. It is very obvious, but I would still point out to the fact that it is a small space. It is good for one person or a couple (although couples or groups of 2 people should bear in mind that the bathroom is very close to the bedroom and there is only a curtain, no doors).
I liked the great use of reduced space and the overall interior design of the place. It is in a very nice neighbourhood, very close to most things in Madrid (I walked everywhere, although I must say I am an avid walker).
Communication with the hosts was good. They are very responsive and helpful.
The only problem I had (which the hosts are aware of and I'm sure will be fixed soon, or maybe has already been fixed) is that the draining in the shower/bathroom is a bit slow, and since it is a small space for both toilet and shower, the bathroom can easily flood a bit. Not too much of a problem if it's just one person, but could be bothersome if someone else wants to use the bathroom after someone has just showered.
One thing that I found very convenient is that, although there is no washer or dryer in the flat, there is a self-service laundry just about one or 2 blocks away. It is not very expensive and it really helps a lot if you are travelling light but staying for several days.