We really enjoyed our stay at Mario's apartment. The location is great, you have many Bars and good restaurants around the corner. Everything is in walking distance, including the Ribeira, Douro river, Gaia (for Porto degustations) , Shopping areas, etc.
Marco offers you vouchers for a nice café in the City were you get free breakfast, which was also awesome!
Be aware that it is an old house, which is nice but the windows don't really catch the noise from outside. So it is a bit loud in the end.