We stayed for 5 nights in July and found Porto wonderful. The best thing the apartment has going for it is its central location on the main boulevard of the arts district. It is an easy walk within 25 minutes to all the main areas of Porto as well as train stations. There is a convenient market 100 yards away and plenty of restaurants nearby.
The apartment itself is of average quality and cleanliness with a well-stocked kitchen. In the summer, it is too hot and humid with no a/c. When windows are opened for fresh air and circulation, flies and mosquitos enter.
It is on the third floor of a building with no elevator, so climbing stairs is necessary. The apartment itself is bi-level and has more stairs. All the stairs were not difficult for us to manage, but if you need accessibility, this might be a problem. Walking around Porto involves a lot of elevation changes anyway.
Beware that if you book an airport shuttle through the apartment's management company and you happen to be delayed at the airport upon arrival for whatever reason (such as declaring and filling out paperwork for lost luggage, etc.), you will be penalized and charged extra by the shuttle service (which is actually just a taxi service).
The Douro Valley Tour is highly recommended and was the highlight of our stay in Porto.