The place is a little old, but has been nicely renovated, and in my opinion, the age adds to the charm. The space is very well laid out with everything you'll need for a short- or long-term stay, and the location is excellent; it's very possible to walk to pretty much all of the historically significant sites over the course of a day or two, though you may need an Uber to reach some parts of the city in a reasonable amount of time.
The hosts were very kind in allowing us to check in before the time posted on the listing, and had a few bottles of water and a small bottle of wine in the apartment waiting for us when we arrived, which was much appreciated.
Hot water in the shower is at least somewhat limited, so if you're a couple, you might need to take shorter showers than you're accustomed to, though it was not a serious concern, and the water seemed to warm again fairly quickly after running cold.
Also, though there is no washer in the apartment, there's a laundromat just nearby.
In short, I can wholeheartedly recommend this apartment for your stay in Porto!