All in all, we really liked the flat.
It is perfectly located to be able to reach by foot the main touristic point of interest, close to any kind of public transportation (visiting Sintra by train was a breeze, catching the direct train from Rossio station).
It's small, but has whatever we needed to enjoy our week-long stay in Lisbon.
The bed is comfortable and we remark that everything was really clean.
We never met the hosts, but getting the keys was easy enough and the hosts were really fast and kind in answering any message requiring suggestions.
The only downsides to the flat, as mentioned in other reviews, are the noise (the flat is on ground level and the trash collection happens around 2 am, so better have some earplugs if you are a light sleeper) and it's a bit complicated to air the flat, both because of its shape (longer than wider) and because the windows are on the smaller side (and again, being on the ground level means not being able to leave them open while out).
All in all, it was surely a positive experience, and the location itself is worth the money!