We spent one week at the house of Teresa & Pedro, and it was really lovely there.
The house was really clean and comfortable. The location is a bit out from the city center, but there is a metro nearby (cheap, clean and quick to get to the center); but you can go on foot, it will not take more than 30 minutes.
Teresa and Pedro were really helpful, advising really good places to visit and eat at. On the first day they also provided us information where to start discovering the city. Unfortuantely we did not have too much time to chat.
The little baby Marina is really calm and quiet, and always smiling. :)

We definitely recommend their apartment to everybody who is planning to visit Porto.