I have used it several times for long-form writing as a critic, rather than as a “co-writer.” I write something myself, tell it to pretend to be the person who would be reading this thing (“Act as the beepbooper reviewing this beepboop…”), and ask for critical feedback. It usually has some actually great advice, and then I incorporate that advice into my thing. It ends up taking just as long as writing the thing normally, but materially far better than what I would have written without it.
I’ve also used it to generate an outline to use as a skeleton while writing. Its own writing is often really flat and written in a super passive voice, so it kinda sucks at doing the writing for you if you want it to be good. But it works in these ways as a useful collaborator and I think a lot of people miss that side of it.
That hasn’t been my experience /shrug