Autocorrect works the way it does to optimize for the average case, and if you type very very fast on the keyboard and ignore any typos you make, autocorrect simply fixes them as you go. I've actually found it one of the better thought out parts of the system. Of course it makes mistakes but in my experience it fixes more than it breaks.
It all comes down to frustration. Having autocorrect "fix" things that aren't broken say 1% of the time, consistently, forever, is a constant frustration that I have no way to remove without either turning it off or changing the way I type (by watching the cursor for the bubble to come up.)
On the other hand, I've noticed simply turning autocorrect off results in less overall frustration, despite less efficiency at first, since the mistakes I make are my own and its my responsibility to improve my typing accuracy so they approach zero.
Yeah, I get irritated when autocorrect automatically "fixes" unusual words I wanted to type, but it's also nice to have correction available. So I'm really happy with ManualCorrect on my jailbroken iPhone - it makes it just correct when I tap the suggestion, instead of automatically. Not a real solution, but nice for my particular case.
