We spent 3 lovely nights in Luis' apartment and, to make a long story short, can fully (even vigorously so) recommend to choose this place if you wish to stay in a perfectly located (albeit quiet, all things considered), lovely decorated and architecturally unique place with a host dedicated to share his love for the city of Porto.
To get into a little more detail: the apartment is perfectly located in Porto's centre, just a few steps away from the next metro stop. As Galerias, Porto's thriving nightlife area is just up the hill. If you turn the other way, the river is not much further down the road. A small supermarket is practically next door, and all sorts of cafés, restaurants and shops are close by. At the same time, it's pretty quiet at night, given the apartment's central location.
The apartment itself is as described and almost as depicted in the photographs - actually feels substantially bigger in reality. Even though it's a one room studio, it never felt packed or too small (far from it). The apartment's clever cube element divides the room perfectly. The apartment has all amenities needed, and was in an immaculate state when we arrived.
Your host Luis is a prime example of what gives the AirBnB concept an edge over hostels and hotels. Luis loves his city and is dedicated to ensure that you will get the most out of your stay in Porto. He provided heaps of useful information, including a digital (Google) map featuring restaurants, cafes, bars etc. Apparently, he is also a tour guide and can provide a private port tasting, if requested. Luis responded very quickly, was absolutely reliable and, above all, is a really decent person.
In summary, this is an imperative recommendation for a great apartment. We'd book it again without hesitating.