Yes, I have used it with languages like Bluespec and Lean v4, which have very little data (FOSS code on GitHub) for the model to work with.

It's nowhere near as accurate as it is when using Rust or TypeScript but it's still mostly "autocomplete, on steroids", just with fewer "oh shit" moments.

I use it for Toit (toitlang.org).

It's not perfect, but still incredible useful (and sometimes scary; in a good way).

I particularly like it for writing tests, but it's useful in normal code too.