My girlfriend and I stayed in the apartment for 4 nights. Overall we had a good stay; the hosts are excellent and the location is great as well.
We did find that there were a few issues. As others have pointed out, the room can be a bit noisy at night. Secondly, the bed that we slept on was hard and squeaky. These two aspects together made the sleeping experience not great. Lastly, we found that the kitchen was not very clean.
Everything taken into account, the room is fine as a base of operations for exploring Porto.