Google claims that it just revolutionized search with Google Instant, which tosses back results as you type each letter. I don't know if it's a revolution, but it's certainly a game changer. I feel confident saying that because it's already giving me seriously mixed feelings.
Google has really been aggressive with implementing change, lately. The most meaningful for me, personally, has been the drop-down search suggestions that help you correct typos or predict what you mean on the fly. Google Instant takes that to a whole 'nother level. (Outside of search has been another story for Google: I'm indifferent to Buzz, though I flat-out didn't care for Wave.)
Now, as you type, Google is continuously updating itself with fresh search results. It isn't instantaneous, despite the name. If you're fast enough, you can hammer out a few words and hit enter before it activates. Even the tiniest hesitation, however, and you'll see websites appearing beneath the search bar responding to your fragmented request.
So, when is it useful? It's questionable. If I want to search for something, say, "StarCraft 2 replays," seeing bland results for "star" isn't going to get me anywhere. If anything, it's going to make the whole thing feel sluggish. Things will start to firm up quick, though, as Google's prediction service seems to be quite the mind reader.
Where Google Instant gets interesting is the fact that you can tweak your searches in real-time. I could change "replays" to "strategies," for instance, and suddenly — without having to hit enter — I can perform an entirely different search.
Really, though, you're only shaving off a few seconds, which is something that doesn't feel all that impactful here. I'll be interested to see if Google Instant ends up being a habit-changer, or if really we're just looking at searching as usual here. It's worth noting that despite how easy it is to switch Instant off (there's a toggle directly right of the search bar), I haven't felt the need yet.
How has your experience been thus far? Are you in love? Have you already toggled it off? Are you going to start using Bing?