Just spent 4 nights at the apartment with a friend and had a very pleasant stay. Centrally located and close to many restaurants, the sights, sao bento train station, and nightlife. We were warmly greeted by Raul who patiently waited for us to arrive (we were late). He gave us some pointers and a quick rundown of the apt.
The apartment itself is a new building surrounded by historic ones. The air conditioning and elevator are a nice bonus. There's a really nice wrap around patio on the second floor of the apartment as well.
If there's anything I would address it's the shower door. It doesn't really have a lip/step at the shower door so if you aren't careful water can get out onto the hardwood floor and from the looks of it, it already has (some water damage). Also, you enter on the bedroom level and the living area is upstairs which is a little odd. These issues were small and overall the experience was really nice. I recommend the apartment if you want a nice place for 2-4 people in the center of Porto.