From the user perspective, this promises to be a very cool interface improvement.
From the search engine perspective, I don't see how this would generate additional paid clicks for each search, so it might not increase an engine's monetization rate. It could certainly pull new users to an engine, thus increasing search volume, thus total revenues. Speed is one of the most important (and most often overlooked) aspects of web usability. (Related post: database scaling)
On the cost side, this enhancement probably quadruples server load due to fetching intermediate results. This looks to be quite expensive at production scale.
On-the-fly search results -- check them out at Yahoo's LiveSearch.