Flat4rents are beautiful and very clean apartments in the center of Porto. They are very comfortable, complete (with a little kitchen), nicely decorated and very clean. We had an apartment at the 4th floor which is a lot of stairs so if you're not in such a good condition, you'll better ask for an apartment on a lower floor. The hosts were very flexibel with the keys when we had to leave, very sweet!
The apartment is in a car-free street but there are some glass containers in front of the building which are emptied around 11-12 pm. That makes some noise. After that restaurants throw there glass in the containers too, wonder why that is allowed in Portugal at that hour.