Paula's apartment is terrific -- it's very clean and nicely decorated. The rooftop terrace is lovely; I wish I had more time to enjoy it! You also can't beat the location; it's close to everything -- at most a 20-minute walk to most of the destinations you want to see in Madrid.
Some things to keep in mind: It is a walk-up to the penthouse (no elevator). It might be a bit of a stretch for 3 people (there is a cot, but not much space; rather perfect for 2 people). The bed is also not the most comfortable (a bit lumpy and squeaky). And the plumbing is some type of low-pressure system that has a loud pump whenever you use the water in the bathroom, which can be loud and distracting if you're not used to it.
Otherwise, the self check-in worked well and we were also able to check out one hour later pretty easily. Thanks, Paula!