Duoro Marina Studios is an amazing place to stay. The apartment itself is modern, clean and has a really nice atmosphere. The views of the marina are great and there's a terrace which gets a lot of sun. The local area is very relaxed - there are a couple of small general stores and a particular grilled seafood restaurant where we ate lunch every day. Paulo was a great, friendly host and even picked us up from the airport.
Porto itself is easily accessible by taxi, which cost under 10€. Or you can get a boat across the river (twice every hour) and use the public transport. Walking there is also possible if you've got time to spare.
Sadly our stay ended a bit stressfully as we discovered a crack in the glass door on the morning of check out. The explanation of the crack is still a mystery as neither party can be sure of how it was caused or whether it was there before check in (perhaps starting as a small crack). Whatever the outcome, this shouldn't deter future guests as it was all-in-all a special holiday.