My Taco Bell had the Black Jacks totally free, no limit. I personally love them, I've probably eaten about 10 of them since the Black Jack came out. I ordered my standard three Black Jacks and a triple layer nacho, minus beans and sauce (which is cheaper than ordering a regular nacho with cheese, and you get more nachos); the total was 89 cents.
I was pissed, of course. To not have known they were free, and order ten of them. I did see some nine year olds go to the register, ask for two Blackjacks, and two cups for "water," and simply get a receipt with their order number.