I've had some success asking it to write me basic elisp functions to perform basic transformations of data or do stuff for window management. It starts hallucinating when I involve 3rd party packages, though.
You have to fill in some gaps (and occasional paranthesis) but it’s pretty darn good at writing elisp for improving your config. I find it’s helpful enough that you can take on medium hanging fruit that you wouldn’t attempt otherwise.
For example, I wrote a minor mode for centering text, an org-export backend for Notion, and some plumbing for chatgpt-shell.
GPT-4 is generally excellent. If you have errors, you can often ask it to fix them. GPT-3.5 is not so good.
I recently asked GPT-4 to write an elisp function that would automatically query GPT using an openai client and insert a docstring written by GPT for any Python function under the point. It worked on the first try!
(I can't assess it myself myself because .el files are at the sorcery level too high for Rincewind-like skills.)