Recently Stanford University released a systematic review of the published literature regarding the health effects of organic foods compared with their conventionally grown counterparts. Their study found that, in general, organic products did not have significantly beneficial health effects. However, of the 237 research results they examined, only 3 actually commented specifically on clinical health outcomes and all of the rest were nutrient- or pesticide-specific studies. The average difference in pesticide presence between organic and conventionally grown produce was approximately 30%, with organic produce exhibiting less residue.
The public has had strong reactions to the study; for reactions to the Stanford review, please see the following NY Times article: http://www.nytimes.com/2012/09/09/us/would-be-healthy-eaters-face-confusion-of-choices.html?_r=2&pagewanted=all.