I worked at a company right out of college that used K for some things. They did some consulting work for the financial services industry early on, and I think that's how they started. I ran through some basic tutorials, but never worked on any of the projects that used it, so never became proficient. Only a special subset of blessed people got to use K.
The only things I remember are that a symbol could could do 3 things when used with a monad, dyad, or infix, and that it was very hard to read anyone else's code because it was so terse.
They also went through a phase of naming their Java classes things like com.a.b after getting used to K's terseness. Fortunately, that didn't last very long.