My wife and I stayed for a month as part of a move to London. Even booking, communication was great. She was transparent, honest, and quick to respond. Once we were there, she was also good about responding to questions and problems.
The flat itself is in a building with an elevator. It does not have a television, but all other amenities. The location is great--only a five-minute walk from Brixton restaurants, shops and the underground station; also walking distance from Clapham and the Overground. Despite being so central, since it is on a courtyard there is very little light pollution coming in, so we slept well!
Problems we encountered: there was spillover noise from the nearby pub when we had the windows open, but otherwise it was very quiet. The couch is a futon and not very comfortable for sitting (the bed was good, though). There are smokers in the building, and with the windows open smoke can get in the flat.
The problems were all minor though and we enjoyed our stay, and Viki is the gold standard of hosts.