We had a very comfortable stay in this apartment. There were a lot of positive sides:
- location is amazingly central
- the flat is very clean, well taken care of & decorated in a really cool way
- the owner is very friendly and helpful: we needed to change our arrival time few times and he was doing his best to help
- there were no overpromises: everything we were told was in place!

few things to consider (as there was nothing really negative):
- while the flat is ideal for 4, 6 is a bit hard to fit as it is a small apartment. in terms of sleeping space it is OK, but having 6 people doesn't really leave space to move around.
- it is on 5th floor without elevators.
- hot water gets tricky when everyone queues up to shower at the same time.

in summary, I would stay again when I come to Madrid, but when we are maximum 4 people!