The apartment is in a perfect location - everywhere is walkable but at the same time it's more residential and not as "touristy". 3min away from a metro station which has connections to basically all lines. In about half an hour train journey you are at the beach, and in less than 20min walking journey you are in the centre of town.
The apartment itself is very clean and has everything you need. The 2 bathrooms make a difference as well!
Carlos is very accommodating and super responsive - great host!
Oh and if you have any doubts whatsoever about going to Porto - just book your tickets already. It's a fabulous city.