The house of Maria del Mar is really good located, if what you want is to visit all the central touristic areas. Just a few minutes of Puerta del Sol and Opera.
However, there where some things considered not acceptable: On the one hand the service given at the check-in by an older lady who was quite impolite. On the other hand, the apartment itself was not very clean (for example, hair could be found in the shower and in the bathroom). The cupboards in the kitchen hadn't been cleaned either. All in all, the apartment looked as if it had only received a "cat wash". After we complained, someone came and cleaned again. The result was sufficient.
Definitely recommended flat if you only care about a “cheap” stay and not service quality. BR Patricia and Hannes.