I could be wrong, but I have a hard time imagining very many people who would get something out of this feature that they don't already have. Search engines already predict what you're going to search for based on what you're typing as you type. Google's page on Instant tries to make a case for the new feature:
Before Google Instant, the typical searcher took more than 9 seconds to enter a search term, and we saw many examples of searches that took 30-90 seconds to type.I can only think of one person I know who might take a minute and a half to type in a few search terms. He types with one finger, and doesn't look at the screen while he's typing. Would shaving "2-5 seconds" off each search, as Google promises, make up for this massive increase in complexity? If you take a longer than average time to search, are you the kind of user that wants thousands of results flashing in front of your eyes and flitting away as you type each letter?
At the opposite end of the spectrum, away from hunt-and-peck typists, I don't think power users will get much out of this either. If I hadn't read about the new feature, I might have never even noticed. I use Firefox's search bar for almost all my searches.
But it is an interesting experiment, and I give them credit for continuing to roll out new features even though their search engine is already really really good. Keep in mind, they're sorting through enormous amounts of information and presenting it all onscreen at a pace that was the stuff of science fiction a decade ago.
