My wife and I stayed in Gary's flat mid-July.
This flat is great - recently renovated, very clean, and more than spacious for our 4-night trip. Unlike some other London flats, the bedroom actually had some floor space beyond just where the bed is.
There's a great view from the living room, and an even better view (towards Westminster Palace and London Eye) from the roof/terrace , which is just outside the bedroom.
There's a fully equipped kitchen, and a cable TV in the living room.
Note that after you take the elevator to the 10th floor, there are a few steps (about 15) to go up to the 21st floor, so this flat is not accessible if you have a stroller or a wheelchair.
The location is good - a 5 minute walk from the Pimlico tube station. So you wouldn't get much in terms of nightlife and restaurants nearby, but you're a 15 minute walk from Buckingham palace, 15 minute walk to Westminster Abbey, and 2 stops on the Victoria line to Soho (Green Park station) or 3 stops to Oxford Circus.
Gary was awesome, he was, by far, the fastest-responding host I've had on Airbnb - He answered all my questions within minutes.
Gary met us, as scheduled, outside the building and gave us all the info we needed for our stay. He was gracious and helped us feel at home. A really great host.