Genetically Modified Species: Harmful or Not?

One of the hot topics on for debate over the past decade or so has been genetically modified foods.
People claim they’re bad for you and others claim they’re bad for the environment, whilst another group argues for Genetically modified foods. I’m going to have to take stance with those arguing FOR GM foods. But first, let me explain:
Bad For The Environment?
Sure GM foods can be more resilient and over-take their non GM counterparts, but surely it’s not too bad a problem if these crops are to be liscensed out to specific farmers. As long as those farmers keep it under control we shouldn’t have any problems that reduce global biodiversity.
Bad For Your Health?
Next, they can’t really be bad for you either, they’re fundamentally based on nature and scientists put these plants through rugged testing and it’s all based on solid research and empirical knowledge. If anything, they are probably better suited to us, if scientists take the necessary measures to get them right.
So, why are people so worried about Genetically Modified foods?