My wife and I had the pleasure of staying in Ana & Diogo's apartment for a whole month while we visited Lisbon, and we absolutely loved it. Check-in was a breeze, communication was always easy and quick, and the apartment was lovely. We arrived to a hospitable surprise; a bottle of wine and bowl of fresh fruit. The apartment itself has fantastic natural light, is decorated very well, and all of the appliances and amenities are updated. It's a small place (my only gripe is that cooking full meals in the kitchen proved to be difficult -- something that most Airbnb guests aren't doing), but the size, decor and atmosphere gave it a cozy charm that we became very accustomed to.
In terms of location, whether you are in Lisbon for 3 days or 3 months, this place is tough to beat. It is a three minute walk from the heart of Graça. Graça is a true Portuguese neighborhood with beautiful color, friendly people, delicious traditional food, and generations of Portuguese families. It is a very quick, nice walk to Lisbon's oldest neighborhood, Alfama, where you will want to do lots of sightseeing (the castle, churches, viewpoints, etc). But when you return up the hill from Alfama, you are glad you are staying in Graça, with its own beauty and charm.
Lastly, on the hosts, Ana & Diogo are extremely welcoming, helpful, and friendly people. Our only regret with our time at their apartment is not getting to know them better. Go enjoy their Graça apartment and see for yourself!