There were no surprises: the apartment was no different than its description on airbnb's site and in the pictures Luis posted. It is bright, comfortable, quiet and very central since it's not more than 5 minutes on foot from Porto's centre.
When I checked in, Luis took the best part of an hour to give me detailed and very useful informations on places to see, sites to visit, museums, the transport system, restaurants, stores etc. It proved very useful for my stay in Porto. I could not dream of a better reception.