I used one when my 330d was stuffed by a 320d (plenty of smiles from passing motorists).
As advertised, they replaced the car for the duration of the repairs with another 330d, took no money off me at any time and recovered all costs from the other party.
However, it took 51 weeks for it all to be finally settled, only after getting to the point of taking the other guy's insurers to court. Standard stalling tactics, but there's the hassle of form filling and so on proving that you needed a car, etc, etc. That was for a straight rear end shunt in traffic with a respectable guy properly covered and admitting everything.
Given the circumstances that you describe above I'd check the small print pretty carefully. As mentioned above there is sometimes a nominal charge (£10 ish) to cover you in the event that they are unable to recover costs, otherwise you'll end up being bent over for them.
As the other party would appear to be playing silly beggars, tread carefully and keep yourself well covered.
Gone: Fiesta - Focus - E46 320dSE - E91 330dMSport - E82 135iMSport - R55 JCW Clubman
Now: M135i with lots of toys