Functionality is, at this time, extremely limited (and the first kfun was out in January so I don't think there's really any intention to get this to usability on a short timeframe). No support for paired syntax like parentheses, functions in braces, and square brackets for indexing and function calls. No stranding or tacit functions. I doubt it's Turing complete. Many primitives are unimplemented and others are flaky: for instance calling count (#) on an atom can give an arbitrary number, print garbage, segfault, or allocate memory until it's killed. But it's got vector instruction support.

If you're looking for a practical k implementation, I recommend ngn/k, and several other implementations are listed at https://k.miraheze.org/wiki/Running_K .