Eurydice - 6:33 pm on Sep 8, 2010 (gmt 0)
Previously, the user typed a search string and then selected the "best fit", even if it wasn't the best possible result.
Now, the user can see the results as she types, so she will modify the search string to get better results until she gets the best possible result.
This will have a major impact on SERPs, ad position, impressions, etc.
All in all, it's a great improvement for the user's experience. Good work, Google!