I can warmly recommend this appartment to everyone! It's very clean and nicely decorated, comes with a washing machine and fast and reliable internet, TV and a small kitchen where to prepare meals. I didn't cook there though as there are plenty of good restaurants within 5-10 minutes walk! And for those who want - Pingo Doce (a bigger supermarket) is located around the corner - just 3 minutes walk.
The appartment itself is located in a quiet street and yet it's very central and close to all the sights - you can easily walk everywhere.
I have been staying for 3 weeks in Porto and felt there already like at home :) Duce is also a very kind host, replies fast and even provides some maps and information about Porto, its sights and surroundings. Check-in and check-out was very easy. Thanks Dulce!