Rafaela and Celio are well-organized and very welcoming hosts. Communication with them is smooth, the check in/check out process is convenient and clear. As others already mentioned, the location is ideal for exploring Porto's historical center, and for going on day trips the train station is nearby as well.
I really enjoyed the interior of the studio, however it was not as well-equipped as I was hoping for. There were some minor inconveniences and just a suggestion: the place could use some
extension leads, so there would be no need to unplug the fridge when you'd like to use the microwave, or place the kettle or coffee machine, or sometimes your laptop to charge on the floor.
Also, if you're in the apartment which is facing the street, it's probably good to note that the bus stop is right under the window, and during night it can be as busy and noisy as during the day.
All in all I can recommend to stay at Ai, Porto!