I'm on my third Prius, been driving them for about ten years, and "saved" about £25 large in congestion charges and road tax as a result. Every four years or so the government changes the goal posts so that Prius is no longer judged to be eco-friendly. So I buy the next one that is.
Quite how purchasing a brand new car every four years to avoid paying congestion charge is helping the planet I don't know.