Quick answer
Is Porto worth visiting at Christmas?
Yes. Porto is worth visiting at Christmas if you enjoy illuminated streets, winter walks, historic cafés, festive city-centre atmosphere and a slower version of the city. It is not as wild or intense as São João, but it can be very beautiful.
The best areas to experience Christmas in Porto are Avenida dos Aliados, Clérigos, Santa Catarina, the historic centre, Ribeira and Gaia. For the prettiest feeling, visit in the late afternoon and stay into the evening when the lights become more atmospheric.