My wife and I and our 13 month old son stayed at David's beautiful apartment for 3 nights in late June 2014. David and his colleague Antonio were very professional and made sure we had everything we needed, even a playpen for our baby to sleep in, which we appreciated very much. Antonio picked us up at the airport and did a nice drive by along the river to show us around before arriving at the apartment as well. He also brought us a map and recommended restaurants and places to see in Porto. David helped us call a taxi so we could get to the train station in no time when we left. Quick review of the apartment:

Pros:
1) Very neat and clean space with a well equipped kitchen
2) Excellent location as it is within walking distance of restaurants, pharmacies, bakeries, and various attractions
3) Blinds for all windows which is great for some privacy and for light sensitive sleepers.

Cons:
1) WIFI is very slow and connection often dropped.
2) I can't really think of anything else other than the fact that the apartment is on a hill and it could be trouble for some people. But really, Porto is hilly in general so it's not much of an issue.

Recommendation:
Stay here! It's awesome :)