We absolutely enjoyed this apartment, the location was perfect for our family: close to everything, the main sights, the Dom Luis I bridge, the train station, restaurants for normal people (read non-tourists), the delightful bakeries of praca de batalha, the tram, the elevator (straight to/from to the Douro caís)...
The size of the apartment was also great, the kids could run around on the hardwood floor without disturbing the neighborhood too much and the kitchen door could be slid close to avoid the accidents while cooking. The elevator was also helpful and last but not least, the temperature inside was cool all day and night with the windows kept open (provided you can deal with the sounds of the screaming seagulls in the night... those creatures never sleep!)
We were there 6 days and enjoyed everyone of them. The closeness to the city elevator meant that when the kids (or the parents!) were tired, we could go from downhill to home in a couple of minutes.
What was great also was that the apartment was across the corner from the beginning of the tourist area, which meant that while close to everything, we could "feel", at least a bit, the vibe of the true day-to-day life in Porto.
Our guest was super friendly in welcoming us. He provided us with helpful tips about places to go, where to buy our food, etc. Here's one if you travel with young kids: right across the Dom Luis I bridge, there's a very nice little park for the kids, in Jardim do Mourro. It's only a 10 minutes walk from the apartment and a great way to have the kids release some steam after having brought them along with you to Porto tasting sessions all day!
All in all, let's say that this was perfect for us, a family of 4 (including 2 kids under 4) and that we loved Porto, probably in good part thanks to this gem of a place!