The good: the terrace, which has quite possibly the best view in Porto. The location is also excellent—central to everything, but on a quiet street secluded from the tourist hordes. Check-in was also very easy.
The not-as-good: as others have noted, the apartment gets pretty damp and mildew-y. The dehumidifier does a good job of addressing the problem, but the tank fills up quickly and needs frequent emptying. It wasn't a big deal for our 2-night stay, but I could see it becoming tiresome with longer stays.
Bottom line: I would stay here again just to enjoy that sublime terrace and view.