The standard OTO chlorine test is simple to use, inexpensive, and easy to find at the store. Still, it has a couple of disadvantages, it measures TC instead of FC & CC and only goes up to 5 (or 3 in some versions). Both of these disadvantages can be mitigated to some extent. The range can be extended with dilution and some sense of FC vs CC can be gotten by watching for a color shift.
I can use OTO to measure TC levels from 0 to 2 to the nearest 0.5 fairly reliably. 3 can usually be made out, but anything from 4 to 8 looks much the same to me. Somewhere above 8 it starts to turn more of an orange, and I have been told that at sufficiently high levels it turns some kind of red. Using a 2 to 1 dilution and measuring levels around 15 that means it would be +-6 or so.
For normal day to day chlorine testing this level of precision is usually just fine. Half of what you want to know day to day is "did my chlorine drop to zero?" and for typical CYA levels around 30-50, a simple below 3, around 3, higher than 3 determination is usually plenty.
I am wondering what other situations OTO testing will work for and which situations OTO testing won't work for. Also, does anyone have experience distinguishing values around 4, 5, or 6 reliably with OTO?