We highly recommend to stay with ORM! These guys are super fast in responding to any message and really make your stay in Porto as comfortable as possible. Alex provided us with some great tips of where to go in Porto, including some non-touristy places.
The apartment itself is perfect. It is right in the middle of everything and you can reach anything within walking distance (incl. a laundromat if needed - 3 min walk). The design, appliances, and furniture are well chosen and in great condition - we really felt like home it's so comfortable! Only one thing to keep in mind; if you're a light sleeper, this might not be the place for you as the apartment is located right next to a busy street.
However, we loved the place and will stay here again when we go visit Porto another time.