I did but it was through II, I think - I don't believe RCI currently has a London DVC swap available. I used TripAdvisor and possibly other review sites to make sure the places I was interested in were good. The closer you can get to a tube stop, the happier you'll be as that will allow you easy access to just about anywhere.
We stayed at the Royal Garden Hotel from the Concierge Collection with points in October. It includes a full English breakfast each morning and free wifi. It's a beautiful hotel right next to Kensington Gardens and Palace. Easy access to the tube. We loved it and felt that it was definitely worth it. Staff is excellent and very friendly and helpful.