The folks over at Yoast posted that they researched the before and after affects after Google extended their snippets maximum length a few months ago.
Their conclusion after reviewing the data was two fold:
(1) That Google often does not use your meta description for your snippet, instead it often uses your content on your web page for your meta description.
(2) In two-third of the cases, Google used sentences from the first paragraph of your content on your page for the snippets they selected in their search results.
Based on this, it appears you should make an effort to have your introductory paragraphs on your pages well crafted for your snippets and users.
Forum discussion at Local Search Forums.
