The usefulness increases with the temperature. It dramatically reduced my printer's consumption when the bed is 95°C, and allowed it to heat up to 115°C without pushing it to its limit (I didn't try anything further).
I have an adhesive insulator from Aliexpress. It has lasted for a few months without any visible wear, but I can't say much about its longevity. But given the price, I would be kinda ok with it lasting only a couple of years.
Why would I ever want to turn 1T into 20T? What difference does it make?
It's much more useful to find some ways to spend 99.99% of it to improve your surroundings. The number on your bank account will still be indistinguishable from the original.
Somebody already posted the famous phrase in another thread: