We have been four nights in Carlos and Clarinda's and it was a perfect stay. The check-in and check out was very fast and easy. The apartment it's near the city center you can walk everywhere and avoid using public transport (BTW metro station is 2 minutes walking from the apartment).
We went there by car and we could park really near without payibg, totally free and just had to walk 3 minutes to the apartment.
I recommend Carlos and Clarinda's apartment if you want to enjoy a beautiful stay in Porto.
Maddi