I booked Sven's place for a few days, planning to spend half of the time in, half out. It was a really nice place. Well designed, lots of nice little touches, really matched the kind of style like.
Sven was fast with communication and left me feeling like if I needed something he'd be there to help.
The flat is in a really good location (as long as you don't mind walking up hills and stairs). It's a few minutes walk from a busy city center with shops and restaurants.
The one issue I did have was there was a weird smell coming from one of the walls which connected to the second bedroom and bathroom. I didn't personally need these rooms as I was staying by myself so kept them shut, but if I had taken my family and we needed a second room this may have been an issue.
Overall, while the smell is still there, I'd definitely recommend this to anyone who only needs one room and bathroom. If something is done about the smell however, I'd 100% recommend this to anyone needing somewhere to stay in Lisbon.