Google Instant is basically a way to show you results as you type, sort of like your browser's ability to complete a URL as you type it in. Estimate: 2-5 seconds saved per Google Instant search and maybe even optimizing your search when you are not sure exactly what you want or how to spell it.
To allay concerns that results for improper words don't show up, like the first three letters of the word 'assign' showing you things you don't want your kids to see, filters are still in place and they say Google Instant is deactivated for slow connections.
Well, Google Instant is also deactivated for a lot of users because it is only available in Chrome (of course), Safari (which is what Chrome is built on), Firefox and Internet Explorer. Don't feel bad, European and Wii and mobile users - they are sure to add Google Instant into Opera some day. It's still new.
Google Instant - for those of you too impatient to finish typing