Pros
If you are a student or in school, it does offer a fairly flexible schedule. In some of the higher volume stores you can make a few extra dollars through commissions on certain products. Store managers are generally pretty cool and laid back.
Cons
You get paid minimum wage, and the commissions are terrible. (Like 10 cents if you sell a certain product.) You will constantly have sales goals to meet. These goals are often geared toward selling GNC's products, which give the lowest commissions, over other products. Higher management (district or regional managers) is constantly riding you about sales. Stores only allow the managers a certain number of hours for employees. This means you will be working alone, opening and closing the store, making bank deposits at the end of the night, and generally being responsible for things that should be a managers job. This also means that you might only get scheduled for 12 hours a week. No incentive program, and no benefits unless you are a manager.