I used it once in 2008 to buy a MacBook. It was fine to use and it works just like a credit card. It will do a credit check and if approved for the whole amount, you will not have anything to pay upfront for the purchase. In one month, you will get a bill just like a credit card. This will happen every month until you pay it off. There is a minimum payment just like a credit card.
However, if you do not pay the entire bill off in full when the 1st payment arrives, you will getting charged at a 19.99% annual interest rate.
Sometimes merchants offer deals where the billmelater has 90 days or 180 days no interest if paid in full by the end of that period, but in most cases after 30 days you'll get hit with interest charges.