Our stay here turned out well, but started with a problem -- the unit we were taken to is not the one we thought we were reserving. There are three flats built out of one, and we thought we were getting the main one, but were shown to another just down the hall. Fortunately, the mistake was fixed within three days (we were there for a two-week stay). So that's good, and I can talk about two units. The good news: Both were sparkling clean, and looked nearly brand-new (there were still stickers on the toilets), with well-stocked kitchen/bathroom and decorated attractively in Ikea-style. More good news: While the main unit faced a busy street, crowd noise was minimal thanks to double-pane terrace doors. The air conditioning was efficient, and welcome. Pro tip, though: There's a bright street light right outside, and the window coverings are minimal, so bring a sleep mask if you don't do well in only semi-darkness.
The bad news: The second unit, the one further in, was very dark with the lights out. The only window was onto an airshaft. And it's not much more than an efficiency, so you can't really get away from your travel partner. With the configuration of it, at the end of a hallway, I worried about fire risk, too. (It had a smoke detector, but I'm a worrier about that stuff.)
But the location is excellent. The Plaza Mayor is right there, as are other Madrid attractions, and the Metro stops are close by, too. We had a great time there.