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.