We had a fantastic stay! The flat is just as cute as in the pictures, and there's a nice view over the rooftops up to the Castelo de Sao Jorge, especially in the mornings. Everything worked, everything was sparklingly clean; we had no problems with the apartment at all. Our hosts were very helpful about check-in and check-out, and left a guidebook of recommendations, which we appreciated. The wi-fi is weak, as others have mentioned, but they've added an extender and we were able to connect to that and get service throughout the apartment. I will note that if you're tall, you might find the bed too short for you, but for us it was perfect.
The location is amazing. It's close to tons of restaurants, shops, and bars, but it's far enough removed from the action that we could close the bedroom shutters at night and get almost no street noise. It's an easy walk to the blue and green lines of the Metro, so getting around was a breeze.