I stumbled upon TFP like a lot of folks -- after getting a stubborn green pool.
While I definitely was sold on the system after the green pool started to clear up in literally minutes, I didn't fully commit to it because I didn't take the time to really understand it. So I'd wind up with a greenish pool just a couple weeks later, which I would easily clear with enough bleach. Not what this system is about.
I finally ordered TF-100 this year and committed to holding my FC up to the right levels based on the FC/CYA chart. It's been clear ever since.
The mistake I made about test kits was presuming that it was about being able to test for different chemicals and conditions (such as testing for CYA and CH). Since I had a kit that could test FC, CC, pH, and TA, I could just take it to the pool store for those occasions I needed to know CYA and CH.
Then I realized that the right test kit is not (just) about testing for more conditions, but about getting accurate, quality results. How could I possibly hold FC at 6ppm with a test kit that only measured up to 5, and even then just gave a range of 3-6? How could I maintain a shock level of 20ppm when, according to my test kit, 6ppm and 20ppm were indistinguishable.
Thanks, everybody, for the great advice and getting me on the right track. Now I'm looking forward to making a contribution.