I had a Kelly Kettle.
It was very fast if all you wanted was to boil water for tea.

You can't cook in them because the spout is too small to put anything in or get anything out.
Using the bottom as a stove after boiling up a jug of water is only a mixed success.
I found out there is just not enough bottom contact with the stove top on a woodstove, the element on an electric stove or the ring of flame on a gas stove for it to work on any of them.
It only works with a little fire burning inside it.

For packing they are bulky with no way to balance that by stuffing them full of other items. That is not as much of a concern if you travel by boat or car but if you need to pack your gear on your back it matters a lot.
They are very easily dented, but that is just a secondary concern for me.
They are pretty expensive here too.

To recap:
They were designed to boil water for tea fast and efficiently with a small fire inside them. They are excellent at that, but they are a single use item. They are not even good to brew the tea in once you have the water boiled.