re: Bond for deed - bad idea?

What would you consider a good chunk? 5%, 10%, or more? Any idea on what the general structure on these deals are? From what I read, they put down a certain amount that they forfeit if they do not go through with the purchase. They also make a monthly payment and a portion of it goes back to them along with their original deposit at the time the deal is closed. Is that pretty much how they work? As far as the current mortgage on the home, I am still the one that pays it, correct? I would not want to depend on someone else to pay a mortgage in my name.