I've never tried Bill Me Later, but if I am not wrong, you must pay a minimum amount every month. if you do not, you get charged a little extra the next month as penalty and it builds up. Also, if you do not pay in full by the end of the duration, you will be charged interest and this interest starts from the first day of purchase (Imagine you bought an Hermes birkin under bill me later 18months. If you do not pay by the end of the 18th month, you get slapped with 18months of interest).
However, if you are confident of paying by the end of the amount and have enough money to pay a little every month, it should be alright. I assume the amount every month would be the full price divided by the number of months. Not quite sure though.