Microsoft released the results of an eye-tracking study focusing on the impact of search ranking, and more provocatively among industry experts, the impact of informational snippet length on user task performance. The early (and already debated) conclusion is that 1. a top 5 rank, not top 10, is crucial; 2. snippet length has a direct impact on search success.
But snippet lengths produced different results depending on the task, according to the 13-page report (PDF):
We found that as we increased the length of the query-dependent contextual snippet in search results, performance improved for informational queries, while it degraded for navigational queries.
In plain English, longer snippets significantly improved the searcher's ability to find what they were looking for with informational tasks (i.e., average June temperature in Caracas). But when navigating for specific sites, longer snippets cluttered up the way.
Our eye tracking results suggest this difference in performance was due to the fact that as the snippet length increased, users paid more attention to the snippet and less attention to the URL located at the bottom of the search result.