My husband and I stayed at Mario's place for a quick 3-night trip to Porto, and it was good for what we needed - quiet, well located, and tidy.
Mario was very easy to communicate with throughout the booking and the check-in process. He allowed us to check in earlier than the stated time as our flight landed in the morning, and this was much appreciated.
The location is very close to the Bolhao metro stop (we took the metro to/from the airport) and the (currently being renovated) Bolhao Market. There are many cute coffee shops and great restaurants nearby (try Bulha!), but it is also an easy 15 minute walk to the hustle and bustle of the water front. We enjoyed the fact that we weren't staying right in tourist central, but could easily get there if we wanted.
The apartment is spacious for two people and tidy. The bed was firm and comfortable and both the bedroom and living room have AC units. There is a back patio, but it looks down into a "dead zone" between the buildings, so we didn't use it. Our only real complaint was that the shower was a bit on the old side; the temperature and pressure were fine, but the door leaked a bit and the shower head wouldn't stay still. The rest of the apartment was fine.
All in all, we were satisfied with our choice considering the cost.