I saw this in the flesh, well metal last week. A garage I had some work done has been messing with it. On a motorcycle there was not any saving but a terrific power surge. However on a diesel car and lorry the savings were great, power improved and the engine ran cleaner, which they anticipated even the dirtiest lorry getting past the London charges.
So it works, the system they showed me was functional but not commercially pretty enough at the moment