We had a great stay at Marco's place.
The place was clean and Marco and Carla were very friendly.
It's in the center of the old-town of Porto. We could walk everywhere en didn't need any other type of transport.
The only 'negative' thing we could think about is that it was a little bit noisy. (We heared the upper neighbours walk around early in the morning). But if that's all..
I would go back there if I go to Porto again.