My wife and I spent 6 weeks living here and it was a great place to explore Barcelona from.
The apartment is in a great location in the small streets of the old town. Despite its central location, it is actually quite quiet after 10 pm most nights. The host Gastón was very good at communicating with us and helped with any questions or request we had.
For a long stay the flat is mostly ideal. My wife worked remotely from the flat without any problem. The internet was very speedy and reliable. The flat has a good dishwasher and laundry machine. The only issue was that the sofa was not very comfortable for watching the TV, but this is a small issue if you spend most evenings out in the city.
Overall, I can recommend a stay here, whether short or long.