I think Google Instant is the best thing to happen to search since "the realtime web" services like Twitter and Facebook added search to their services. These two things are not directly connected - I simply see them both as giant leaps in how we will find information for many years.
The speed at which I can now find information using Google is simply stunning. Sure, many companies can do incredibly cool things. But, not many companies can do incredibly cool things a billion times a day.
Because of this speed increase I've now set Google as my default homepage in Safari. I've never done this. Ever. My homepage had always been the Top Sites feature in Safari or Chrome, some news site or a blank page. But now, because I can search the entire Internet nearly instantly, Google.com is my homepage.
On the Mac I use Spotlight to access nearly everything on my local computer. I also use it to do tasks like math or opening applications. Spotlight is a fairly versatile tool (and yes, there are more robust alternatives) to helping me both find and interact with pretty much everything on my computer very, very quickly. Google Instant is like Spotlight for the entire Internet. Do math, pull up a map, check the weather, see movie showtimes in your area, look at beautiful images of Yellowstone, and see the latest news without moving your mouse.