A high-quality receiver is the brain of your home theater and should last you a decade if you pick the right one. Buying a durable unit now means you won't need to replace it when technology shifts again in a few years.
Wait until February to shop for last year's models. Manufacturers drop new gear at trade shows in January, which forces retailers to clear out the previous inventory at a discount.
The 18-month intro window is a deadline, not a gift. If you still have a balance once that time is up, you will start paying interest on whatever remains, which makes the receiver much more expensive.