Yes, the rooms are amongst the cheapest in Madrid and the location is good, but...
in this location there are actually 3 (or even 4) hostals, one on every floor, sharing reservations (without telling their clients?). They even seem to have contracts with other hostals in the street. Chances are you won't get the room you reserved at the Martin, but one in a hostal on another floor or somewhere else in the street (in a hostals, of which you haven't read the ratings).
This can lead to the following situation: I arrived at the hostal, but no room, despite reservation. A flustered front desk guy phoned a guy from one of the partner hostals, then rushed down which forced me to scramble after him down the stairs with my (heavy) suitcase. After waiting at the entrance 10 minutes in vain, where the other guy was supposed to pick me up, the hostal martin guy ordered me back upstairs, because suddenly there was another room available in that building.
A minor detail giving some insight into the strangeness of the place: Apparently the guy at the front desk is working a whole night shift every night in that room with the bright head lamps and a loud constant beep (reminded me of "Lost") every 5 seconds enough to drive anyone insane (The alarm system, apparently).
A not so minor detail: Turned out the room was only free for one night (despite reservation), so I would have to switch rooms. What do you do at 12 at night, with a checkin procedure already lasting 40 minutes and a business date tomorrow at 8. You shut up and go along with the madness. "When you are in the room, call me if you want the air condition turned on". "What?, turn it on now", I said (It was one of those bathtub nights with 32 degrees). I regretted that reply 6 hours later in a freezing cold room, while searching for a nonexistent switch on the AC box. What could one possibly do to an AC that would prompt a hostal owner to remove any setting mechanism. Took 20 minutes under an admittedly well functioning shower to warm me up again. The thing about the TV (besides having no remote), is that it was so big, it blocked the entire surface of the only table in the room, so your suitcase has to go on the floor. And then there was this strange living room next to the entrance that looked like someone's grandmother had died in there, with a huge messy pile of dirty towels on the sideboard that staid there untouched 2 days. Creepy.
Why ruin your belief in mankind by staying in a weird, disorganized place like this. Try some of the hundreds of other hostals in the area, all with great location, all with absolute basic prices, most of them with better service. May I recommend the hostal Cervantes or the Hostal Principado, just as two of many more examples. (This tip is not hearsay, I stayed in both of them)
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC