Good: The apartment itself is very nice and clean, and communication had a pretty quick response time. Location is also good, although the closest metro station is approx. 10m away.
Bad: The biggest issue was that the air conditioning was not working when we arrived, even though the listing said it would have AC when I originally booked it. We visited Madrid in July, where it is over 100F every day, so having AC was our biggest priority. Also, the host didn't notify us in advance--I actually checked the Airbnb listing while I was packing for my trip to see if the apartment would have an iron, and that was when I noticed that AC was no longer listed. I had to contact the host myself, and it was only then that he told me it was broken. I expected it to be fixed by the time we arrived, and that never happened. Very disappointing and made our stay much less comfortable than it would have been otherwise.
Also, it was nearly impossible to get into the building/apartment with our keys. We stood outside of the unit for nearly 15 minutes trying to get it open. We eventually figured out that you have to put the key in ~just right~ and turn it just the right amount for it to open. Not the end of the world but was a recurring inconvenience throughout the stay.