We stayed at Helena's and Artur's place for three nights.
Their apartement is very nice. Decorated in a mixture of modern and old style with a beautiful stone imitation wall. Very well equiped as well - from TV and kitchen utensils to a washing machine. Even a hair dryer was there, despite not being listed at airbnb.
Not only was the apartment great, but Artur, who let us in, was the type of host you could only wish for. So friendly and welcoming. He showed us the apartment but also spent more then half an hour showing us on the map what to visit and giving us valuable local tips as to where to eat. He said he was at our disposal for any questions and tips. Really really friendly.
The only "downside" of the apartement is that the internet connection is not the fastest one, but it works and believe me, Porto and the surroundings are so beautiful that it for sure will not disturbe you.
We felt really really good and welcome at Helena's and Artur's apartment and if ever again in Porto (hope so :)) will stay at their place again.
Obrigado! :)