The apartment was conveniently located to metro stations, and also within walking distance to all the major sights. They provided a complimentary bottle of wine, as well as a couple beers, bottled waters, and colas in the fridge. There are grocery stores in the immediate area, so we were able to make a couple of our meals while staying there. They provided all the basic amenities, except for perishable items, so remember to buy some olive oil, salt, pepper, etc if you plan to cook in the apartment.
Miscommunication in English with our host led to a bit of a misunderstanding and hiccup at check-in: We thought check-in was at 11, but he meant he never gets the apartment until 11 to clean, so we ended up having to wait a couple hours. But they offered to store our luggage until we could get the apartment, so that was nice.
As someone else mentioned, there are signs taped on the doors and walls with some house rules, contact information, as well as warnings to not throw anything into the electric toilet (not even toilet paper), so that detracted from the ambiance a bit.
Overall, it was a decent place, and we were able to enjoy our stay in Madrid.