I've given El-Tesoro a 5 star review as I had a great time there. I booked the day before arrival and was delighted at the quality of the apartment. Its clean, beautifully decorated and has everything you need for a short break in Madrid.
There are two key things you need to be aware of to avoid disappointment. These weren't an issue for me which is why I've rated 5 stars but they could be for others.
1) There are a lot of stairs with no lift. It's not easy carting luggage up these stairs, I'm a male in my early 30s and reasonably strong.
2) The walls are paper thin. The owner considerately leaves earplugs for you which is a nice touch but if you can't sleep with earplugs in you will likely have an issue.
Neither of the above is an issue for me and if you are like me then you will be delighted with the apartment.