My girlfriend and I had a great stay. The apartment is incredibly cute and charming. So is the neighbourhood. It's quite quiet overall, even though there's a lot of cool restaurants, bars, cafes and stores nearby. And you can walk pretty much anywhere from it.
One thing I'd say is the bed was a little too hard for my taste. Some people care about it, some don't. I do, that's why I'm even mentioning it since I wish I knew it beforehand. Still, not enough of a reason to make our stay there any less wonderful.
Guillermo's friend, Antonio, was very nice. He let us in before the regular check-in time, explained to us how everything works, and made himself available for whatever we needed (even though we didn't need anything).
Overall, the experience was marvelus!