Autocorrect is a bad idea. The kind of spelling error that iOS corrects (At least as far as I know.) is a syntactic error. Whereas the kind of error that autocorrect introduces is a semantic error. A semantic error is much worse than a syntactic one. At worst, a syntactic error leaves your meaning ambiguous. At worst, a semantic error leaves your meaning incorrect or inverted.
Usually you can read past a syntactic error, but a semantic error could change the tone or meaning of your entire message.
Well, it's a question of total value, weighted by frequency. If the autocorrection gets ten syntactic errors fixed for every one semantic error it introduces, it's probably a net gain for the writer.
Have a little care, then. You have to be careful anyway with a touch keyboard. Autocorrect makes me faster, by correcting most errors without making me stop, and that's all I ask of it. I can't recall a time it's stung me in a major way; I can't count the number of times it helps me every day.
Usually you can read past a syntactic error, but a semantic error could change the tone or meaning of your entire message.