I am actuially a personal trainer part time and work for a reputable hosp as a bill/collector full time.
I came back to the med industry after about 2 years of personal training... it was to unstable for me (but I live in Arizona) a draw back was the weather differances and snow birds comming and going and a good part was how much money is here to milk.
I just know personally from expierance I am happier here at my reliable desk job these days. It was great when i was younger the easy money and flexable schedules but now I need something more perm.
Just be careful where you end up getting your expierance at and you'll eventuially want to go free lance but you will need insurance, which can usially be obtained from onc of your certs. I believe I got mine through NASM or NCSF back in the day.
I would reccomend trying it out first and seeing what you think of it first before you dive head into it, it's not for everyone and it's a very tough start building your client list and keeping them when your a new trainer. expierance is the key. If you have a true passion for it don't get discouraged at first as most will.
and good luck!