I recently rented a flat through Black Katz and have been very satisfied with their service. I went to a lot of agents but Black Katz were the most responsive and helpful. The only thing that I thought they can improve on is their service when getting into the flat; to come in the flat with you when you first move to show you a few basic things (e.g. how the heating, rubbish etc works).
But overall I was very satisfied and I would definitely recommend them as it is indeed hard to find good letting agents in London.