The location is amazing - right in the middle of Madrid, right on the Gran Vía. You're in the middle of everything. The flat is incredibly spacious. There are multiple balconies with great views, and you get great views from many of the windows (especially the bathroom windows haha). Flavie is responsive and helpful. There's also a Carrefour right around the corner for easy groceries (and of course a million nearby places to eat and drink).
The check-in process was a bit of a mess. I think there was supposed to be a doorman to help us in, but there was no one there, so we had to randomly ring doorbells in the apartment building until someone let us in. It was a bit stressful, and nothing in the check-in instructions had mentioned that part, so we were really confused about how to get into the building. After that, though, the instructions were clear. Some of the information in the flat was a bit out of date (the garbage cans weren't where they said they'd be, the wifi info was different, etc). But Flavie was responsive to all concerns so it got resolved quickly.