Personal training can be a lot of fun. I was a personal trainer for a couple years back in 01-02. It is very rewarding to see your clients reach their goals. Reaching physical goals does so much for people: empowers them, makes them feel good about themselves, more energy, more focus, etc.
I made some money, but not much. Difference was I worked for a gym, where I only made about $27 out of the $80 a session the gym charged. $27 an hour sounds good, but its very difficult to schedule your clients back-to-back, and it can be hard to have enough clients to fill a 40 hour week.
I also worked under a lot of restrictions, things I wasn't allowed to do, and things I had to do, whether they fit in with the clients expectations, wants or needs. Eventually, I stopped enjoying what I was doing, and in training, if you are not happy, either are your clients.
If I could do it over again, I would freelance. Most certifications will include quite a bit about legal requirements, marketing, an general information for the freelance trainer.
My advice would be to get your certification and work at a gym for a bit for the experience, and then freelance. Unless you can find a gym that is trainer friendly, but I haven't found one yet that works well with trainers.
Thats accually a good thing for me to know, I was thinking of doing that as a side job in the future. Now I know what to expect.