Pros
You actively see the changes that you are helping to make in people's lives. You form a great connection with clients and become a foundation for their improvement. The job is very rewarding, and if you can manage to get enough clients in your schedule, the pay is decent as well.
Cons
I'm a student and the hours were sporadic. At the time that I was still a trainer, each client would average 4.5/week with you. At most, I only ever had 3 clients at one time, and I had to depend on them to not cancel so that I would get paid for those hours. There were times when the scheduling left an hour or two between clients, but I was too far from home to make good use of the time, so I would sit in an unused roomed and try to work from my phone or laptop. You also don't get paid for the amount of paperwork that you need to fill out each session. You get paid for 15 minutes per session, but oftentimes it could take closer to 45 minutes to do paperwork after or before a session. It was also common for trainers to get word that they had a new client with less than a day's notice. If your schedule is not up to date at all times, you run the risk of getting a client placed in any time slot that is not already accounted for after 6am. No room for advancement.