I spend 6 days in the apartment and and my stay was really pleasant. I was welcomed by a very nice lady who provided me with some tips for sightseeing. The apartment is well equipped and very cozy. It was indeed a little cold during my stay, as others stated before. The heaters provided did in my opinion not have enough power to make the place warmer, only if you sit straight in front of them. An electric heater with more power would do a better job here I think. There was also some mold in the bathroom which is probably a result of the rather cold/moist apartmentduring winter, I guess in summer it will be probably gone. I'd recommend to keep the bathromm door closed (and always open the window in the bathroom) so you won't breath spores when you sleep.
Besides those two flaws I would really recommend a stay here. I think in summer the apartment is a super nice place since the concrete will keep the heat out.
It's a little uphill walk when you want to get back to the apartment but since the food in Porto is super tasty, a little workout can not be wrong ;) There is a small grocery store when you enter the door to the left side and walk straigth down the little hill of the street, there you can get 6 L water for less than 1 Euro so you wont have to drag it up all the way from town.