good idea in that you are getting the money at 0%. Be careful of the terms however. Some schemes lend for 3 years, equal monthly installments with no final baloon payment at the end. Others might only give 0% if you can come up with a minimm of 50% payment up front, ususally in form ot trade in.
bad idea, in that you are not the owner of the car until you make the final payment, therefore a trade in before the final payment is a little more complicated