My old thermostat controller was not working; my spa was either too hot, or not heating resulting in it being too cold.
I finally did the online research and selected this item. It arrived quickly, and after opening up the heater, I installed the thermostatic controller in just a few minutes. With only two screws and two electrical connections to connect the process was easy. It was a little harder to pull the thermostatic bulb sensor out and reinstall the new on, but a pair of pliers did the job. Make sure not to damage the copper strip that secures it in place or bend it out of shape.
After installation, the heater cycles through spa too cold - turn on heater to spa hot enough - turn off heater.
My only gripes are that it seems a bit expensive for such a simple device, though spread out over the many years I expect it to work makes it economical. The other gripe is that I like to be able to dial in a hotter temperature, this tops out at 106º and I like to go up to 110º.
All in all, I'm satisfied with the product.