I've been using it to give it the rulesets or general concepts to video games I'm designing and asking it for ideas for special powers or cards, or providing it with an overall theme and asking it to generate ideas for how to apply that theme to the ruleset or game concept. It provides a lot of junk ideas, but enough useful ideas that I keep asking it. Something another game designer said that I agree with is that it's 'about the same as a playtester tossing out a random idea.'
You have to evaluate it to see if it fits your goals, and work out the details and prototype and playtest it, but sometimes just seeing those ideas can get you unstuck or enhance the design or spark another idea which works even better. And some of those ideas I know I would never think of, and probably never get them from a playtester, so I'm thankful for them.
Also used it recently when my wife was feeling stuck in the motivations for certain people in her book, I gave it a paragraph overview and asked it to come up with motivations and it provided quite a few, including about six or seven we had previously tossed around and discussed. She was skeptical beforehand but was more impressed afterwards, in particular because it provided a bit of validation that the ideas we were tossing around weren't bad ideas.
Just this morning I asked it for what tools within Azure cloud tech to use for accomplishing a programming task, and the information it provided me seemed a bit more useful than me asking other experienced users in our organization as well as a few videos and articles I read (because it was able to provide an idea more directly applicable to our specific use case). Even provided some sample code.