I stayed for 4 weeks (month of March): Diana and Sergio were very gracious hosts that were happy to accommodate each of my requests.
The apartment is spacious, modern and within walking distance to everything you could possibly need. It's clean and in a safe neighbourhood just minutes away on foot to the city centre.
Only slight downfall was the noise of their children above running in the house. The noise stopped by 9:30pm and didn't cause much disruption.
I would definitely recommend this place for a long or short stay in Porto.