Mario's place on a third-floor walk-up was spotless and located right in the heart of things, while simultaneously being quiet. Check-in was easy and Rafael from Guimaraes (I believe that was his name) was professional and helpful.
Be aware that the terrace faces the back of a bakery. So, there is no view and the terrace smells faintly of bread, which is not necessarily a bad thing.
Despite that it's an excellent, clean/spotless apartment perfect for a couple. The amenities inside the apartment were a tad spartan, but the mattress was super comfortable. And again, the apartment was extremely quiet.
The apartment is also right next to Food Corner, which has about four or five restaurants serving burgers, sushi, pasta and veggie choices at affordable prices. And yet, inside the apartment we did not hear any of the noise coming out of Food Corner. Antiga Leitaria about a block away was probably the best meal (Portuguese tapas/petiscos and also sweet) that I had in Porto.
The Bolhao Metro is about two blocks away. The airport via the Metro is about a half an hour away. The Sao Bento train station is about a few minutes walk away. One can easily walk all over the Baixa without taking any public transportation.