We were very happy with Marco's apartment. The location was great, the old town and most of the sight-seeing were in walking distance. The apartment was very clean and fully equipped: kitchen with pots and pans, extra blankets, even shower gel and basic cooking ingredients (oil, salt etc.). The price was reasonable in comparison to how much other accommodation costs.
The area can get a bit loud; however, that's the way it is in the centre, it all comes down to your priorities.
We arrived late and left early so we didn't get to meet Marco but the communication worked perfect and he was very helpful with everything we needed.
Highly recommended, we would happily stay again the next time we are in Porto.