Last week i have read a very Good article on Google webmaster Blog that "With Google Instant, you may notice an increase in impressions because your site will appear in search results as users type." so i i have check and monitor my analytic but i did not find any changes in result that means Google Instant help ful only those site which are very popular in search engines.
The impressions they are referring to are those detailed in the Google Webmaster Tools, in other words it increases the impressions in the search results
I don't think this update has effects on rankings, it's just some new features on showing serp results but don't change ranking positions..