I used 9flats.com several times over the course of the last year for both business and leasure trips.
The booking process was really easy and I really like the concept of showing the total trip price and not being charged extra in the end which is the case with many other travel sites.
The best thing are the prices: You can stay in a central location without feeling squeezed in a tiny room, enjoy free wifi and all that for 50% less than in a hotel.
Connect with Facebook to get one click access to Trustpilot. It's easier.Sign up with Facebook