Isa's apartment is just great! We loved its old style, the spacious sunny balcony, the bathroom was equipped with everything that you could possibly need :)
Isa is a lovely host, very friendly and responsive. We missed our train and we arrived two hours later than planned, but she was waiting for us and replying to all my messages right away.
Even though the place isn't at the very city center, there is a metro close, so no problem with that. In fact, the location was a reason we just walk more and see more of the city, not just the touristic spots, which I think was great.
I strongly recommend Isa's place to anyone who is planning to go to Porto! :)