In my experience it is not so good that you could happily swipe away without needing to review the generated text. For example, sliding to type "rife" autocorrects to "ride," despite the context of the sentence making "rife" the only possible word.
With the caveat of Apple Intelligence is disabled, I believe it is that good and that's how I generally use it: with reckless abandon. And it does a great job. YMMV.
I believe that with AI turned off, it's not using LLMs for autocorrect -- or at least not the new Apple Intelligence LLM junk.
For me, typing on an iPhone worked well until iOS 26. Now I get jumbled AutoCorrect which acts like it's LLM-driven.
When I turn off Apple Intelligence on iOS 26, I get the old, properly-functioning AutoCorrect back. So that's a potential workaround.