I didn't read any replies, but it comes down to salesmanship--period. Stick a used car salesman on a gym floor, and he'll probably have more clients after a week than all the other trainers. Ask yourself honestly, am I a good salesman? If you're not sure, don't bother in my opinion.
The job has its perks, no doubt, but it's generally a tough living. No paid time off, no sick days, scanty benefits. Get the flu and cant train for a week? Hope you have some reserve cash saved up.
I did it for 2 years, had fun and learned a lot about myself but ultimately found it to be a dead-end job and left it for a career offering more financial stability.
Some people absolutely make it work for them, but they are few and far in between in my experience.