I find the autofill useful and annoying in equal measures.
When I'm doing what the authors consider "normal" things, such as filling in screens in Amazon or eBay, it works a treat.
When I'm filling in search fields, it gets in the way, believing that I really want to search for me, my email address and my postcode. Luckily starting to type over the given name normally clears the rest of the fields.
It's not half as annoying as when Ancestry fills in (unbidden) ALL the search fields using a random person from my online tree who happens to have the same given name as the person I was intending to search for. Say I am looking in the 1861 census for William Henry McIntosh, born Scotland around 1830 and living in Kent. I don't find him as William McIntosh, so I click "Edit Search" and change the first name to Henry. Ancestry then leap into action and change the fields to search for Henry Higgins, born Leicester, Leicestershire, England 1877, died Leicester, Leicestershire, England 1878, father John Higgins, Mother Mary Wilson. Cancelling the Edit correctly returns to the search results, but Ancestry have now remembered the abomination, and Edit Search now returns to the "Henry Higgins". Of course they fill in fields I would rarely use so the only safe way is to clear the whole lot and start again. I know there's a "clear" somewhere at the bottom, but I find it easier to close the tab and start again from scratch.