We stayed at the Nook in Bairro Alto for five days and enjoyed ever second of it! The flat was easy to find, tidy and very well equipped and most of all extremely cozy. Since it's located in Bairro Alto, next to quite a few bars, there are plenty of possibilities to go out and enjoy the nightlife of Lisbon. If you are an early riser this might not be the ideal place for you to stay though due to the noise coming from the bars at night. But we did not mind and were very happy with our choice to book this apartment.
What made the stay especially pleasurable and memorable was the hospitality of the hosts: The communication was great from the start and not only did we receive very detailed directions on how to get to the Nook, we got a lovingly put together list of recommendations for places to go to and things to do besides the common tourist attractions - including a whole Fado playlist to get us into the proper Lisbon mood :-D
We felt incredibly welcome and safe and would definitely recommend staying there or at another apartment of José, Paulo and Catarina!