Pros
As a personal trainer for over 5 years at Gold's Gym, I can say that there are most definatly some advantages. Setting one's hours is especially convenient, that has really been the biggest selling point for me all these years. Hourly pay: i.e. you work it, you get it. If you're willing to grind day and night, no one can stop you from taking 25 clients per day and raking in the money.
Cons
NO benefits. None. Zero. Also, no uniforms, you have to buy and have them printed yourself. Tasks. More and more tasks are being asked of us as part time workers. Clearly defined in the roles of the personal trainer are to keep the gym clean. But there is nowhere in the payment system to allot for this time. So it ends up being free labor for them, and frustration for us.