I've never actually used it. Deactivated it on every smartphone I've ever had, and never looked back. I write in French, English and Frenglish all the time and I doubt autocorrect would keep up.
For the most part I like autocorrect. It lets me touch type on a screen based keyboard. It slowly learns the fake words I type and even suggests it later when typing, but sometimes it replaces correctly spelt words with a completely different word and then does it again and again when I try and fix what it did.