Criticisms of the KK come from people who have tried to use them as a stove and been disappointed.
They are a portable wood fired water heater, not a stove. And they do that job better than anything else I have ever seen. My Kelly Kettle will boil 1.5 litres of water in 4.5 minutes
from lighting. Normally people will let a fire get going before they pop the pot on and begin timing.
The next closest I have ever seen is my own stove, which if you allow it to get going before you pop the kettle on will boil 1 litre of water in 6 minutes. And it was melting the handle of the kettle even managing that.
The kelly kettle is in a league of its own.
I've been wondering if a neglected application of the Kelly is as a water purification system for expeditions. It gets the water
really hot so I'm confident no bugs could survive in there....
...oh yeah and I'm pretty sure it would make an awesome still!