If you are an independent Spanish speaker, the Mirentxu can be a great value option. I used to live in Madrid, so I knew it was in a great location that was safe and convenient to all the major sites. I had a single without bath right near the front desk. The proximity was not a problem since the front desk was rarely staffed, and guests are given all the necessary keys upon checking in, so one can come and go at will. The older woman who is there most of the time speaks no English, though some of her younger relatives do.
The room was spacious with basic furnishings, and a wash basin with only cold water. The single bed was adequately comfortable. Heating was fine. There was only one electrical outlet in the entire room, though the hotel has excellent free wi-fi. It looked like there might have been water damage at one corner of the ceiling. There were french doors that led out to the air shaft/courtyard that is so common in Spanish buildings. Since my room did not face the street, it was very quiet unless, as happened one night, one of the other tenants in the building decided to have a loud late night party.
There were two shared bathrooms (one with toilet, shower, and washbasin; one with just toilet and washbasin) just across the hall from my room. Both seemed to be relatively renovated and were spotlessly clean. There were no lines or long waits to use them during my visit.
Probably the biggest drawback to this hotel is the lack of air conditioning, which would be a severe drawback in the summer. And, at peak occupancy, there might be more wait times for the bathrooms and/or increased noise in the hallways.
So for a Spanish speaker who already knows Madrid, staying during the late winter off-peak season, this was a great bargain for 28 euros/night.
One tip: People have written about difficulty finding this hostal. It is on the 3rd floor of the same building as the La Ancha restaurant, famous for its paella, where a meal can cost as much as one night's fee at the hostal! Every taxi driver should know it.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC