Like any loan, the question is whether it makes sense given your savings (if any) and what interest that savings is earning (if any) versus the interest of the loan (if any).
In this case, it's basically letting you collect interest for 90 days on the money you'd spend on your Mac. If you're buying a tricked out top of the line Mac Pro and you have your money in an account earning 5%, that's almost $200.
If you're buying a $599 mini, it's almost certainly not worth the bother.
I bought my wife a watch this weekend (Zenith Chronomaster Open El Primero, for you WIS's) and the store gave me 6 months same as cash if I signed up for their credit card. Someone
is going to make interest on that money for the next six months. I figured it might as well be me.