Fantastic location to explore and fantastic city!
We stayed here for 5 days in March and enjoyed every moment.
The apartment is in a fantastic location, 2 minutes from the riverfront and not much further from the cathedral and bridge. It is spacious, light and very comfortable.
At night, as you are in the centre, there is some noise from the streets outside but it didn't cause us any harm and what else do you expect when you're in the middle of the city.
Transport routes are great with the metro a 5 minute walk.
When we stayed there was building work happening next door which is completely out of the owners control. If this sort of thing bothers you (although it didn't really affect us much other than not leaving food uncovered in kitchen) then take that into consideration if you're staying within the next month or so.
We also had the odd problem with running out of hot water in the shower but again this wasn't a major issue just something to be conscious of if you, like us, take longer showers.
Overall, this is a fantastic flat in a stunning location that I would highly recommend to anyone wanting to visit Porto!