I haven't used Google search for much lately so I haven't really noticed this before.
Today while searching with someone over the phone, I would type in a phrase and before I finish the search phrase, Google auto completes and then drops me into the search results which are not what I was looking for.
I then have to go back up to the search bar and complete the phrase I initially started to type.
Have to say I'm not at all impressed with Google trying to think for me and causing me to do more work which is why I also think their so called "usage" is up according to that one engineer.
I saw this happen yesterday on a Windows pc with Firefox. I hate autocomplete and just ignore it. So I typed my phrase and hit enter at the end. Since autocomplete had jumped to the serps at some point, my enter was interpreted as a click on the first link and I found myself on a web page I never intended to click. Did I just get turned into a zombie?
Autocomplete on my Android devices makes google search almost impossible to use. The keyboard keeps going away and the screen goes through all kinds of gyrations after every character is typed.
Autocomplete is also an area that goes through update after update without much notice from webmasters - but it certainly can affect your search traffic. If your target query is part of the suggestions, then moves up or down (or off the list) even though you ranking is the same, your traffic can be affected profoundly.
Autocomplete also seems to be localized in a very granular fashion, making it pretty hard to be disciplined about your observations of this factor.