Sure, it changes the way we will search. But is that a good thing?
It's all about whether we want change. Change can be good, but think about how long (if ever) it could take for more time to be saved than time lost getting used to the change in the first place.
Also think about it this way: people have been searching on search engines for over decade in an very uniform way, regardless of the search engine. Search engines aren't that intuitive, even if they try to be. They aren't that user friendly, even if they try to be. But people worked with what they were given, and got quite good at. Now Google wants to break our habits, and a I don't think I want to. Not only because of the time I would lose changing my habits, but because it works just fine.
Another reason why Google Instant does not work for me is since it only works for "browsing" and not "finding". Sometimes people will go to Google to just "browse" a broad topic. They're not necessarily looking for a specific website or detail. So after entering a broad search term, Google Instant helps them narrow their search query. On the other hand, sometimes people will go to Google to "find" a specific topic subject, point, or answer. Therefore they don't want suggested search terms. They have already crafted a detailed search in the heads and are not looking for suggestions. In these cases, Google Instant just acts as a distraction. And that brings me to my problem with Google Instant: I use it for "finding" 90% of the time and only 10% of the time do I use it to "browse". I have a feeling that many tech-savy people here on HN probably have a similar distribution of their time spent on Google Since Google Instant only works for us the 10% of the time, it's hard to justify using it all.