I can never decide on whether a KK is useful or not. On one hand the requirement for an actual fire for anything is excellent but it never seems more useful than a hobo stove or Jetboil unless you will only ever be making drinks and even then it's quite sizeable.
I'm not knocking them, just musing.
They're superb for doing what they're made to do, boil water.
There's nothing which comes near them for speed and efficiency in my experience.
They are a quite big and bulky though but they weight next to nothing which is good.