A sharp portrait lens gives you that creamy background blur and clean subject separation that kit zooms just can't touch. We want to buy glass that lasts through two or three camera body upgrades so you're only paying for it once.
Watch the manufacturer release cycles closely this spring. When a brand drops a mark-two version of a lens, last year's model instantly drops in price even though the optics are nearly identical.
Remember that the intro window is a strict deadline and not free money. If you still carry a balance when those 18 months are up, standard interest kicks in on whatever is left over.