Used Hailo while on a night out in Croydon (yeah yeah, was already taking my life into my own hands ;) )
Told it where I wanted to go, and it gave me a quote.
It told me it would be there in 9 minutes, told me the drivers' name, and the reg plate of the car.
I had the choice of paying up front (I had already registered my debit card), or paying by cash in the cab.
The cab arrived - it looked like an ordinary cab but with a Hailo-enabled mobile on his dash.
Cab was decent and didn't take any tourist-routes.
To be honest, all I wanted was a cab/taxi to take me from A to B, without having to wait outside A and hoping a cab came past.
Yes, I could have rung a mini-cab - but I never have a number handy, and I hate sitting in people's private cars.
I don't see any reason why the French taxis couldn't also subscribe to the Hailo-like system, although I'm sure they'd have to pay a cut of their profits.