We spent two nights at Fernando Y Guille's place, and overall I'd say it was a good experience.
We never met either of the two, but were provided clear self-check-in instructions. What wasn't obvious to me from the AirBnB description was that this wasn't a private listing, but more of a professional guest house (this seemed to be a business, with more than one apartment on the floor).
This wasn't an issue for us, but if you're looking to stay at a lovingly decorated private residence with personal touches, this might not be your cup of tea.
The apartment was bigger than what we needed. It has a kitchen, couch, and a TV in both the living room and the bedroom. We just stayed here to sleep, so we can't really comment on these aspects of the apartment.
In terms of location, the apartment is well-located. It's not dead center, but you can conveniently get everywhere (incl. city center) by foot. We walked everywhere. Sevilla isn't that big.
When we were there (January) it was a little chilly, but it was fine. Just remember to turn on the heat as soon as you get back, as it can take a while (and I believe you can't leave it running; there's a mechanism where if you leave, the electricity turns off). For very light sleepers, I'd recommend bringing some earplugs. The bed was also OK, but if you're above average height like me, it might seem just a little small for longer stays. All in all I slept well during the two nights we were there, though.
Communication with the host was fine; we'd usually receive a reply within 24h or the next day.