It was a great stay at Isa and Juan's place, and Isa was extremely friendly and helpful. She waited for us at the apartment late at night and gave us every detail we'd need to stay and travel around.
The location of the apartment was superb, and was only a few minutes walk from Pl. Espanya transport hub.
One imperfection was hot water. The boiler had a very small tank, which was barely enough to finish even one shower. This may be less of an issue during warmer seasons, though.
Another caveat is WiFi. The two networks provided at the apartment were quite slow and unstable, so be prepared to use your mobile data / tethering if you have important things to do online.