We stayed one night at the Maydrit Hotel, near Barajas Airport.  The hotel has much to recommend it.  The price was very reasonable - - a little over €100/night for a super clean, good-sized, modern room.  Breakfast is included, and the restaurant serves up quite acceptable lunches and dinners.  A free shuttle to or from the airport completes the package of features.
The best thing, however, was the friendly and helpful staff.  The day before we were to catch a flight to Casablanca, to meet up with our tour group, I received an email from Iberia Air that our flight had been canceled.  The included phone# gave a recorded message, in bullet speed Spanish.  I know enough to catch the word for unavailable.  I asked one of the men at reception for help who told me that Iberia will actually have seats available for passengers with special circumstances.  He had a number to call, and got through immediately.  He explained that we were needing to meet up with a tour group; and, voila!, a seat was found on Royal Maroc Airlines, leaving just an hour later than our original one.
Throughout our stay, all staff were similarly kind and helpful.
I second this recommendation. We usually stay here the night before we fly out of Madrid. If you have time, the subway station is a short walking distance from the hotel and you can get most anywhere on the subway. They have a pool and a decent restaurant. Nice, clean rooms and a friendly staff.
Sounds like a find - thanks for posting about it!