It was a good stay, and very true to reviews. The owner speaks no English or French but is a very nice guy and somehow makes communication possible. The apartment is spacious and functional, only a short walk down the hill to the river, 15-20 min to the pedestrian bridge. Gaia side of the waterfront is quieter and feels less touristy than the Porto side, bars and restaurants are nice and feel a bit more local.
One minor downside was the slightly incomplete kitchen (one tiny baking tray, no foil, no colander, etc) so if you plan on cooking, I would recommend checking if the necessary equipment is there before buying your groceies.
Overall, great place to stay.