This was a real gem, which we thoroughly enjoyed, and highly recommend - it is walking distance to nearly everything in old/scenic Madrid (we even walked to Reina Sofia, but taxi'ed back from Prado). On weekend evenings, the Puerta del Sol is a kaleidoscope of people and street performers - highly entertaining. If you want to take the Metro, all three Madrid lines go through Sol station, and it's easy to hail a cab there day/night.
A few things we wish we had known up front:
1) the key pick-up is a few miles away, so we were lucky and accepted the airport "transfer" from the property manager (emailed a few days prior to our arrival). Airport taxi is a flat 30 Euro anyhow, so for an extra 10 we got a driver who knew where to stop for the keys (we never found a Madrid taxi driver who spoke any English, so getting one with instructions "pre-loaded" was a big plus), and didn't pay extra for the stop/wait at the management office.
2) The apt is just off Puerta del Sol, which means the taxi only can get you within about 3-4 blocks; make sure you can move your luggage this distance without assistance.
3) Ignore the street-level view/door; the apartment was on the 4th floor and quiet as a tomb, despite being so close to a bustling street scene. Elevator is smooth and holds 2 passengers + luggage easily
4) A few things took getting used to - the door has a multi-bolt "bank safe" design, and you need to turn the key 3+ full turns to fully unlock it (we were close to despair, until we figured this out). The WiFi name/password is printed on the box behind the sofa. We never figured how to work the induction range, but the microwave, washer, and dishwasher were simple enough to work, and we were grateful for all three; there was laundry/dishwasher detergent under the sink to use. The cable TV has a host of Spanish-language channels, but nothing in English.
5) If you need (literally) anything, walk across the street to the Corte Ingles, go down one flight, and there is a huge grocery store with good prices and tons of Spanish wines. We stocked the fridge cheaply.
6) You can Yelp restaurants in the area; we found a good one on Calle de Bordadores (El Neru).
I wouldn't recommend it for four, but it is large/luxurious for a couple, and we would return in a heartbeat, if we are in Madrid again. My sole recommendation to the owner would be to leave 1-2 pages of English instructions for the appliances (stove/range, in particular).