It usually isn’t much good at writing new code from scratch. You have to be so specific on what you want that by the time you fully described the code you need, you could have written it yourself.
What it’s really good at is refactoring or finding bugs in existing code. I will frequently paste in some ugly function that I’ve written and say “can you make this more readable?” and 100% of the time it produces clean, readable code that’s nicer than what I gave it.