During our stay in Porto we enjoyed our stay very much at the Cristal Apartment. The apartment itself is beautiful and the balcony is excellent to spend your siestas on. But as with every stay, there were a few minor issues. The location is not really in the 'actual' center with all the nice buildings and restaurants, it is still 5/10 minutes walking. Next to that, we had some problems with the shower getting hot due to the boiler. In the beginning they give some instructions to fix the problem, but it didn't really help.
But in general, we enjoyed our stay very much. Oporto Rental Management took great care of us and were really helpful. Porto is a beautiful city and the apartment itself is great.