Pros
Not very hard. Commission is a nice plus, but don't count on making mad money, especially at a smaller store, and chances are you will start at a smaller store. Extra hours can be had easily because most stores have so few employees, when someone has a problem they will call in a mad panic trying to find someone to fill the shift. At a slow store, some nights you can go for hours without a customer, so you can play gameboy, do some homework, or whatever you want.
Cons
Pay is never a cent above minimum wage unless you're a manager. You get a teeny raise once a year, when they get around to it. You are pretty much a used car salesman, and they make you dress and act like one. You have to pitch overpriced tribal medicine dressed up in fancy bottles to customers who give you funny looks when you can't tell them how a product works because they think you're a pharmacist when actually you're getting paid less than they guy at the McDonald's drive-thru. LOL. If you think of customers as "suckers" who are easily taken advantage of, GNC might be your calling. To move up to management, you don't need to be competent, just have good sales numbers, which is often more luck than anything.