Just between the cathedral and Sao Bento station, next to the Louis I bridge the place offers one of the best locations for a short stay in Porto. The place is nicely furnished, comfortable and very clean. Nice view from the balcony too! No dedicated parking (to be expected in the city center), but free parking available on a small street by the apartment.
I can definitely recommend staying with Paula & Rafael, they were very nice, helpful and responsive.