I'm not a blanket opponent of GM, but beyond the silly "Frankenstein food" rhetoric there's a real argument about the limitations in our knowledge of the potential side effects; particularly where unlike cross-bred bananas, plants are being "enhanced" with genes purposefully to make them toxic to certain species. We're not really in a position where we can confidently state that a gene will always behave in the same way when introduced to an alien species and left over a few generations, or that a particular "harmless to humans" protein will continue to be harmless to humans when consumed in higher volumes as part of a staple foodstuff. Testing ameliorates that risk but doesn't remove it altogether.

Obviously similar arguments apply to our lack of real knowledge of the side-effects of pharmaceuticals and vaccination programs even after substantial testing, but there's an argument that the rewards for developing cures for disease are worth risking the side effects in a way that increased crop yields aren't. Moreover drugs tend to directly affect the individuals taking them, whereas 'enhanced' crops and their non-sterile wind-blown pollen affect an entire ecosystem.

Then again the alternatives seem to be less-than-risk-free pesticides anyway...