Martin is easily the best host I've ever had the pleasure of meeting; he's energetic and warm, and took the time to give us a few cultural tips (like when lunch/dinner times are!) as well as personalized recommendations. You can tell he genuinely loves Barcelona and wants his guests to have a great time as well! Because of the location, we were able to walk most of the city, and take busses and trains further out—all extremely intuitive and convenient.
We stayed for about a week and though we weren't in the apartment very often, we did notice the lack of natural light the longer we were there; just noting in case that's a strong preference! That said, the apartment was nearly pitch-black at night (lovely). It's on the main floor, so we heard some lobby/elevator noise, and a lot comes through from the upstairs neighbor. Nothing a pair of earplugs can't fix, but something to be aware of.
Overall, a fantastic stay in a wonderful neighborhood—very much recommend!