Karen's place is in a good location in Madrid, you will be close to metro stations and grocery stores. Its a nice place with a good layout, comfortable bed, equipped kitchen, and quite a lot of potential. However, there are definitely a few areas for improvement. For the record, they are new to the Airbnb experience so I am guessing as they get more experience hosting and many of these kinks will work themselves out. Airbnb experiences tend to be pretty consistent and this one was more like a short term rental situation as we were there for a full month. The neighborhood is charming and nice, you will be close to everything either by walking or metro stations near the house (Embajadores or Tirso de Molina).
There were a few things we wish we would have known up front. For one, the whole time we were there, there was heavy construction going on in the building every weekday from 9AM on almost all day, making it hard to sleep in during the mornings, get work done, as well as causing things like water & electricity to go out here and there and the need for workers to enter the apartment. Its probably useful to note that Karen isn't really the main host, there is a local host who was SUPER nice and very helpful. He is awesome and did a good job of trying to deal with everything but all the commotion and day-to-day things that naturally spring up from construction work certainly caused interruptions.
The internet was very unreliable and would go out (the longest was for nearly a whole week) so if you are planning on working in the apartment that can be risky. It was especially stressful because there were important work calls that sometimes had to work around the internet issues. Totally not a big deal for a vacation but if you are renting this place longer and need to get work done it can be a real problem so just be aware.
In terms of comfort during the summer, we recommend the host investing in is some more powerful fans. During hot Madrid months, there is not even a full sized floor fan to cool the place down. There was a tiny fan for the entire apartment and the extension cord was super short so it wasn't even easy to position it in a way where it helped much. The apartment gets really very hot which will only matter for the really hot months in summer. That would be the number one improvement once all the construction and such comes to an end. Overall we really like this place and think that with a few added improvements it will be a top notch Airbnb experience. :)