Approved Car Finance operates along the same lines as Yes Car Credit, which was forced to cease trading after a report on the BBC Whistleblower programme. i.e. they offer credit to people who can't normally get it but charge high prices for their cars and have extremely high interest rates.
The ACF website quotes a typical APR of 39.9%. With NatWest and RBS both offering credit cards with 13 months interest free credit, with an APR of 13.9% thereafter, it would make far more sense (for those that can do so) to look around for the lowest prices on cars and then finance it from a credit card (or bank loan).
The only reason for using ACF would seem to be if you really can't get credit elsewhere. Even then, it would probably be more sensible to buy an older (but far cheaper) car for cash. For example, I've just bought a Mondeo, which looks and drives like new, with a full MoT, for �350.
Chris