Mario's place is in a very central location, just a minute from the Metro at São Bento, so it's very easy to get to from the airport. There are some great restaurants and wine bars nearby, and the riverfront Ribera area is just a ten minute walk. Mario was very understanding when our flight was delayed, and explained the location of the various interesting locations in the city. Porto itself is a great place to visit with some amazing, historical architecture, especially from the Belle Epoque style. Prices for eating out are very reasonable, and transport is great. Only downside to the location is the noise, as the apartment is above a very busy street, with lots of late night noise, but other than that, its fine for a short stay. Wifi works OK, supermarket and bakery nearby, and Mario is easily contactable.
For a great, stylish restaurant, try Cantina 32, just a few minutes down the street, at 32 Rua das Flores. Also for a very cheap but delicious lunch, try A Sandeira do Porto Rua dos Caldeireiros 85. The place is tiny, but the staff are lovely and their sandwiches are great. For later in the night, go further up the street to Miss'Opo, Rua dos Caldeireiros 100. Hip and trendy.
Enjoy Porto, it is a fun lively place, with lots to offer!