You want a pair of noise-canceling earbuds that actually block the train rumble and last for years, so you only buy them once. A good set makes your daily commute quiet, but paying full price right when a shiny new model drops is usually a waste of cash.
Tech companies update these lineups on a predictable cycle. If you wait just a few weeks until the newest version hits the shelves, last year's model drops in price while keeping almost all the same performance.
Remember that the intro window is a deadline, not free money. If you still have a balance left when those 15 months are up, interest charges will kick in on the original amount.