I had a campaign last year that netted me $45,000 but recently Google changed its algo or manually edited the autocomplete for my sector. It's a neater subset. But I don't think I'll bother in the future. I assume the algo will have some special scrutiny for every major autocomplete phrase if it doesn't already.
A friend of mine did some intensive testing of the auto-complete function (also called Google Suggest). His report is that the search suggestions are intensively geo-targeted, showing different choices in different geographies, sometimes even down to the zip code level! That's going to be difficult to take into account if it affects your chosen terms.
One thing is certain - if your chosen term disappears from the Suggestions, your traffic usually takes a big dip even though your rankings stays the same or even improve. This effect can be seen even if your query term stays in the Suggestions but just goes lower on the list.
Sometimes Suggestions are the first place to look when diagnosing traffic troubles.
This is a really interesting way to look at it Tedster, i almost never track that but have seen a lot of traffic drops without ranking drops so I will have to pay more attention to the auto complete in the future.