Best Buy's 18 month financing?

I want to buy an iphone because the phone I have is starting not to work to well. I want to buy the iphone 4s from Virgin Mobile & I found out about the 18 month financing plan that best buy has. This plan would be perfect for me because there's no way I would be able to pay $400 something all at once. So if anyone has ever done the 18 month financing from best buy, how did it work? & is it a good idea for me to do that.

Have you ever been late paying a bill? Then don't do it.

There's no interest IF you pay in full in 18 months AND are never late. If you are late, even once, then the interest rate of about 25%-30% is charged going back to the first day. That will end up costing you an extra $100 or so + $35 for the late fee.

They offer these deals because they know many people will screw up.