This is amazing--though Amazon will give a 30-Day Free Trial to practically anybody with a pulse, and they offer a free Six-Month Prime Student Trial for any college students, we don't often get a true discount on a Prime membership. And today they're taking off $20 so that your first year of Prime is only $79 instead of $99!
However, this promotion applies ONLY to new subscribers. Fortunately, you are considered to be a new subscriber if you are currently in a 30-day or six-month free trial. But you'll have to start your paid year of Prime today. So if you're in a 30-Day Prime Trial, it's definitely worth it. If you're nearing the end of your Six-Month Student Trial, still worth it. If you just started your Six-Month Student Trial, this isn't really the deal for you. You don't get to pick up the rest of your trial at the end of your first paid year.
It's important to note that this year subscription will appear as $99 in your cart. It is ONLY once you proceed to checkout that you will see the discount applied and the $79 price. This is a confusing process to some new to Amazon Prime, so make sure you're looking at the checkout page and not just your cart before you complain that the price isn't working for you. And if all else fails, contact Amazon directly. They're always incredibly helpful.