When we visited Lisbon it unfortunately rained a lot, so we spend quite a bit of time relaxing in the appartment. We really enjoyed how spacious the appartment was, and the comfortable couch and beds! The kitchen is very big with all necessary equipment. During our visit (end Feb/ early March) it was a little cold without central heating but luckily there was a little electric heater and there were little blankets to keep us warm.
The location was very nice too, just outside the busy centre, with the market accross the street and various little shops and cafés around. The metro stop was very close too, taking us to the centre in 10 minutes.
One minor point of attention; sometimes there would be quite a strong smell coming from the larger bathroom. Other than that we really enjoyed our stay in Happy Flat and we are definitely planning to come back another time in the future!