I've *just* taken delivery of mine (the 1 Pint - smallest) size and been thinking to recommend them here - great topic!!!
It's a brilliant idea and very well made. It's about as simple a design as it get BUT it works really, really well.
For a bush-crafty boiler, it's about the best thing going. All the mess of cooking the water over the fire-pan is pretty well contained inside the kettle and so it's not bad for that. It doesn't require a lot of wood. In fact, we want to process more wood than the Kelly Kettle needs.
It's aluminum, but reasonably thick. I plan on making a steel "base" to go on the inside bottom of my fire-base, but thats just me. After contemplating getting the stainless steel one (and waiting 'til they're shipping out), i bought the aluminum one and am actually very pleased with it. It's light and great heat transfer with the aluminum - faster cooking!!!!
Since most here are based in the States, you can check 'em out right here:
http://www.kellykettleusa.com/kelly-kettles.html
The one i have holds 20oz, but it's better to leave a little space for the boiling so i used a (measured) 16oz - half of a Nalgene.
I've been e-mailing the owner of the Company and he's been excellent to work with on getting my Kelly Kettle to me (i live in a harder-to-get-things-to country).