My two friends and I just spent 2 nights at Marco and Carla's apartment in Porto, and loved it! The apartment is so charmingly decorated, and was very spacious with 3 bedrooms.
The location was perfect for us. A short walk from the train station, and walking distance to lots of great shops and restaurants. It's also close to all of the port lodges on the other side of the river - I highly recommend walking over the bridge, just for the views alone.
All of our interactions with Carla and Marco were prompt and pleasant - they were very nice hosts. The apartment was very tidy and extremely affordable, and served as the perfect "home-base" for a couple days of exploration.