Fantastic little apartment right in the middle of all the action! I would stay here again with no hesitation.
The apartment itself is very nice - 1 bedroom with comfortable queen size bed and plenty of closet space; bathroom with shower; kitchen with fridge, sink, stovetop (no oven), microwave, dishwasher, clothes washer (no clothes dryer), and plenty of dishes and cooking utensils; 4-person dining table; living room with sleeper sofa and TV with all the channels you could want. The living room and bedroom both have separate air conditioning units. My favorite, however, is the little balcony overlooking the street and all the action. You can even see part of Plaza Santa Ana. Even though the apartment overlooks the street, the windows are very well insulated so you will not hear any noise from the street while you're sleeping, especially if the a/c units are on. Windows have shutters to block out the light, if you wish.
The building itself is very nice and clean - elevator, easy stairs, trash receptacle on the ground floor, secure entrance. I rarely heard any of the neighbors the entire week I was there!
The location couldn't be better - 5 minute walk to the three major museums, 5 minute walk to Puerta del Sol (metro, Renfe), <10 minute walk to Plaza Mayor or Retiro Park. There are TONS of restaurants in the area (literally steps away) and several grocery stores within just a couple of blocks.
Random Madrid tips in no particular order: 1) If you're going to the grocery store, bring your own reusable bags or they'll charge you for bags (plus it's just the environmentally responsible thing to do). 2) Don't be tempted to by a multi-day metro pass - Madrid is a very walkable city and other than to/from the airport, I didn't use the metro once. 3) Allow 1 hour to get to/from the airport by metro or Renfe - Sol is the nearest stop for that.
Enjoy Madrid!