For people discovering k for the first time, I wrote https://xpqz.github.io/kbook as a gentle intro.
As far as I can tell they are best for data analysis but not for heavy numerical computation because most or all of them lack GPU support. is that right or have I got it wrong?
Are there any other use cases?
regarding use cases, see https://kx.com/resources/use-cases/
q is good with bulk operations on compact arrays; these are cache-friendly and the interpreter can utilize cache-level parallelism. and with q it's convenient to go from idea -> MVP in short time. it's a high-level language with functional features so expressing algos and complex logic is natural.
but it's interpreted and optimized for array ops. so really latency-critical (e.g. high-freq trading) or highly scalar logic will be done with C++. the trade-off is convenience of development.