This is SUCH A CUTE APARTMENT. I was only able to stay for a night as I was on a long layover, but I would definitely have stayed for longer. The apartment is gorgeous (warning: in summer, since it's a loft, it will be hot. But there's an AC and opening the windows was really helpful) and the neighborhood is super quiet. However, the apartment is located on a street corner so if you're a super light sleeper bring ear plugs.
The location is within walking distance from the center of Porto — maybe 20-30 min walk to the main retail area, and another 10 to the Duoro river. Getting to the apartment from the airport was super easy and Sergio was very communicative and flexible with my late check in. Overall, I'm very glad I chose this apartment instead of staying in the airport for 18 hours :)