Ok, so this will be my first tricky review on Airbnb. Tricky because our overall experience in Madrid wasn't great for many different reasons, and only some of them were related to this apartment. So I will try to give a balanced and fair review.
We stayed in Montse's apartment for three nights. Upon arrival, we needed to self check-in ourselves (usually no big deal), which turned out to be the biggest challenge of the stay. One of the keys provided was totally bent and wouldn't open the apartment door. We had to carefully straighten it inside the lock. Suffice to say that this also could have broken the key altogether. Despite the information that it was a high security lock, the key was soft and easy to bend - seemed low quality.
After we managed to enter, we quickly discovererd that we also had issues with the key for entering the house from the street. It only worked once for me and then never again. Here's where I want to highlight that Montse was very quick to answer and found a solution for us. We exchanged our key at the end of the first day and had no more issues with the keys afterwards. Still, these keys shouldn't have been handed out in the first place and it's the reason for not giving 5 stars.
More about the apartment: The apartment has a fully equipped and new kitchen, a super new and clean bathroom, and everything looks like on the photo, though it is a bit smaller than I imagined from the pics. As stated in the description, you are sharing it with another party, which can obviously be fun, annoying, or not really make any difference. Our only issue was that our flatmates showered for very long and there was no more hot water when we wanted to shower. It's therefore important to wait some time between showers so that the water can heat up again.
The location of the apartment is perfect when you love the Lavapiés and La Latina neighborhoods and don't mind that it can be loud on the streets at night. Not any issue for us, but it's probably worth mentioning that due to the noise, it's better to keep the windows closed.
The communication with Montse was great, she always responded within a few minutes. She also had some nice tips on where to go for breakfast and bars.
Finally, would I recommend this place? Yes definitely - but only when proper keys are handed out. A note regarding the shower would also have helped.
btw: there were no cats in the apartment, which may be good news for cat allergic people (we would have loved to be greeted by a cat).