Staying at Cristina's was my best experience with Airbnb so far.
The flat was very pretty, modern and very functional. There was everything I needed for my stay over the half marathon weekend.
Location was perfect, right in the city center, 20 seconds away from the seafront, yet a very quiet street which I was grateful for, having to sleep early before the race the next day.
I was warmly welcome, and Cristina had very good recommendations for what to do in town.
If you ever have to stay in Porto, by yourself or with a partner/friend, I highly recommend this place.