I stayed a whole week in this apartment and enjoyed it very much.
Luis welcomed me personally, showed me around and even gave me a list of recommended places (restaurants, bars, etc..) to visit in Porto.
The apartment was very clean and has a comfy character. Nothing feels used up. The kitchen was very useful for a quick breakfast or preparing snacks since it has basically everything you need.
The bed was comfortable and there are power outlets up there, which is always useful.
The apartment itself is located in the inner city, but in a rather quiet street with no traffic (cars or people), so you can actually get a good rest without being woken up every minute. Just around the corner you will find two cafés (with yummy pastries) and even a supermarket. The metro station "Balhao" is just a five minute walk away and the station " Trindade", from where you can reach the airport, is just a ten minute walk away.
Porto isn't a big city, so everything is pretty walkable (if you don't mind walking up and down stairs or hills). For example: I reached the cathédral or the famous Sao Bento station in about 20 minutes.
I recommend the apartment to everybody who is either traveling alone or a couple. I will definetely go back here if i ever visit Porto again.