Ever since one of our 747's was hijacked about 15 years age the operator I work for is required by law to carry a minimum of 3 armed air marshals on each flight (the only exception is flights to and from the USA - US FAA requirement). These personal fly on full fare tickets paid for by the government, hence no loss to the airline.
My opinion as to their value is mixed as there is no attempt by these personal to be covert. They in fact stamp their authority on the pax by doing a final carry on baggege search at the door as the pax board.
There instructions if something was to occur is confidential (even from the Captain) but I can assure you that since Sept11 the policy is now much more robust.
I suppose you would have to say the policy is successful as there has been no attempts since these personal have been carried.
Will the system be successful here, that is a 64 dollar question. I guess it all depends on the perceived threat.
Have a nice day