Nicely renovated, immaculately clean apartment. It was advertised as 20sq.m but in fact I think it was a bit bigger than that; certainly plenty big enough for 2 people. Comfortable bed, nice sofa, modern well-equipped kitchen. Effective, quiet aircon/heater units. Good WiFi. Room serviced daily. NOTE, all the apartments are up at least 1 flight of stairs; there's no elevator.
Excellent location handy for metro station (direct trains to/from airport, 35 mins journey, every 20 mins) just beside the Mercado de Bolhão - but note, the old market building is being renovated in a big project and is closed until 2020; many of the vendors are now housed in a temporary location close to Bolhão metro station. Plenty of interest nearby - Via Caterina shopping, tiled church, useful restaurants. The Ribeira (river, quayside) is a bit further - about 20 mins walk.
We got a breakfast-inclusive rate, good breakfast provided in the brilliant Cristo Rei bakery/cafe about 50 metres away.
A couple of very minor niggles: (1) checkin was easy, but a little fiddly as you have to go to the owners' other apartments about 200m away; (2) disappointing that there were no basic supplies in the kitchen (oil, salt, pepper etc) - it's kinda tedious to have to victual the place from scratch if you want to do any cooking, although it's perfectly possible because there's a good cheap supermarket around the corner.
In summary: would definitely stay here again if visiting Porto to see the old town.