It's a decent studio in the perfect location. I'd say it's a bit pricey for what you get but the selling point is the location.
Great communication with the host(s).
Beware that check-in time is 5-6pm. This didn't bother us, as that's around the Time we arrived in Lisbon. But keep it in mind if you're arriving early, as there's no option to keep your luggage (like in a hotel).
You can tell that they did their best to give it a unique touch (e.g. the hand-painted decoration on the wall). We didn't mind that it was very small and the bathroom tiny.
Something else I guess the owner can't do much about is a strong unpleasant smell when you first enter the studio. The best I can describe it is the smell of humidity and old furniture.
The biggest downside for us was that there's no bathroom door. There's a curtain instead but it's not helpful as you hear everything. Hence, don't come here with company if you're not super close, if you know what I mean.
Finally, it could be that it's because it's a very old place, but there were a couple of bugs on the wall, on the kitchen table, and the floor. It didn't motivate us to use the kitchen.
If you're not planning on spending a lot of time in the room (like us), it's fine.