Hi, Rhonda:
I did it a year and a half ago. The home study course is good but I live in a rural area and couldn't really go to any workshops that many others have said were helpful. I studied for about 8 weeks and passed the test which was hard, but not killer. If you love the material, you will do well.
How marketable you are probably depends on you and where you live. Not too many people in rural northern California have money to throw around for a luxury like a personal trainer, but I have some people I help. I can't imagine eking out a living if that were my only income. However, the knowledge has been extremely valuable to my job as P.E. Tech in the local high school.
I hope Honeybunch will chime in here, because she really dove right in and started using her certificate in a way that got her some great experience right off the bat.
I got sidetracked by emergency back surgery and rehab, and then I was afraid to go out on my own because leaving my job which has poor pay but great benefits was not something I could face. For me, in the long run, just being able to help more people even casually has been worth it. I'm even excited about doing my continuing ed credits.
If you are braver than I am and live in a relatively densely populated area you will do great. Good luck!
--Ann