The location is perfect. 3 minutes walk to the heart of Lisbon, close to parks to walk at night, cool restaurants and bars and yet in a friendly and quiet neighbourhood. Just amazing.
The flat is small but it has everything you need really. But you're in Lisbon so you'll probably just need it for sleeping ;)
The only thing I see is that if you're not travelling alone or is not a couple (let's say you're with a friend or whatever) you might have some privacy issues as the bathroom is all opaque glass so not really visual and sound proof =D
Other than that, it's clean, pretty, check in and check out was excellent and communication was top notch.