Things I think guests will love:
the view is glorious
the location is great with less than 10 minutes walk form the metro from the airport, and to the Torre dos Clérigos & Sao Bento train station.
There were some excellent bars & place to eat nearby.
The bed is firm & comfortable.
There's the basics to make your own breakfast- kettle, microwave, fridge - there's also a couple of hotplates I didn't try. Alternatively Mario provides vouchers for breakfast at a choice of 2 cafes - which is some fresh bread, cheese & jam, & 2 drinks - a good start to the day both places I tried it (available between 9-12). The cafe in St. Catarina were also really kind & accommodating in storing my luggage on behalf of Mario.
Mario was gave me all the information I needed & was prompt at sorting out a problem with the shower
Things to be aware of:
as the apartment is in the city - there is some noise, not intrusive but if the sound of a car driving on cobblestones will wake you up- bring some earplugs.
This is great, relatively simple accommodation for staying in Porto & an absolute bargain if a group of you are staying