We stayed at the music flag for 6 nights. The apartment is located in a quiet portuguese neighborhood which is great for someone who wants to experience more than just the touristic parts of Porto. There are plenty of bars and cafes within walking distance. It is also very reasonably priced and clean.
It is a good size for two people but the walls within the apartment and in the building are paper thin and it is very easy to be overheard and to hear things you might not want to :)
Overall it was a convenient and pleasant experience and definitely worth recommending!