I can't imagine a better location for exploring Porto and Gaia. The view from the top floor overlooks the river and has an incredible view of the skyline of the old city of Porto. It's a short walk to all the restaurants on the Gaia waterfront, up the hill to the metro, or across the lower span of the Ponte de Luiz I to the Porto waterfront.
The apartment is well kept, comfortable, and modern, and has all the amenities we needed, including an iron and ironing board. With big windows on the river side and street side, you can get a cross breeze that keeps the apartment comfortable all day and night. (There's air conditioning and a heater, but we didn't need either for our visit in early August.) We were pleasantly surprised by how much storage space there was: big shelves in the armoire where we could stash our suitcases, lots of hangers for our clothes, empty drawers under the bathroom sink for our toiletries. Despite being a compact city apartment, the space was very usable and never felt cramped for just us as a couple. It might be doable with a couple and one or two kids, but any more (or any more adults) would not fit, at least not in the third-floor apartment that we rented.
The host is very communicative and gives great instructions. As listed, there was no washing machine and no on-premises parking, but the host gave very clear instructions for getting to the convenient free parking around the corner, and we were able to come and go as we wished, even late at night.