We are living in Lisbon for 3 months and took a 4 night holiday in Porto. This 3rd floor walk up flat is right on the Douro river with a storybook view out the bedroom. The view is amazing! There is a convenient car park nearby and Luis met us there and walked us to the flat, which was nice. He gave good info on where to get food, an overview of the area and described and demonstrated everything in the flat. The kitchen and bathroom and heaters all work fine in this quirky and quaint Apt. It is ideal for a short term stay. The bathroom is small and the bed is very hard.
You could stay long term, but the odd layout (bedroom one side with a door, then a hallway with stairs wide open to the middle of the building, then bathroom and kitchen on the other end of the hallway) mean than, in addition to walking up 3 flights, you actually walk through the hallways of the lower 2 floor apartments, one of which has tables and homey stuff that you have to dodge around to get by. That stuff belongs to a life long tenant, an older lady whose daughter is on the ground floor, so you hear them talking loudly up and down the stairs, and her yelling down to anyone who comes into the building, you hear her on the phone, etc. That, and dodging her stuff every time you go in and out would make it tough as a long term stay. Fortunately, being on the 3rd floor means that nobody walks through your hallway, so you have much more privacy, and, since we have dogs, we could let them go back and forth from the bedroom to the kitchen.
You walk out your door to the riverfront with abundant restaurants, boat tours, boat taxis to across the river, etc.