Very nice flat in a pretty central and quiet area. We stayed for a month and had everything we needed within walking distance.
The flat itself is very nice, kept well maintained and clean.
There is a comfy sofa, and TVs in both the bedroom and living room. The only drawbacks with the flat are that the bed is a little bit small (for me at 191cm) and the TV's are a bit old with poor sound quality.
The area is good and feels very safe. However you are right above a car mechanic shop, which can get quite loud during the day - this isn't a problem unless you are planning to sleep in late.
Check in and communication were very good, we also had a cleaner come every week which we didn't expect and was a lovely touch.
Overall can definitely recommend for your trip to Porto.