Very highly recommended! I would definitely stay here again if I returned to Lisbon.
Positives-
Excellent communication from friendly hosts who are very helpful and prompt.
Excellent amenities in the very large studio/flat. Toiletries, everything you need in the kitchen including toaster/coffee maker etc, TV with satellite.
Clean and new, very well decorated. Comfortable bed and sofa.
Very central. Close to the metro. Close to the start of the trams. Close to shops. Close to restaurants (Cantinho Aziz, the Mozambican, was amazing). Close to the old town (Alfama) and castle.
Negatives-
Only a couple!
Regarding the flat itself it would just be the tiny windows that are at one end only. It gets stuffy and you miss the natural light. Personally I didn't notice the noise but it might be a problem for some very light sleepers.
Regarding the area, it seems a bit dodgy at first sight. There's a lot of rubbish around and it's quite dark at night. But I think in reality it is perfectly safe and it's so convenient for everything.