I found this article on why genetically modified crops won't feed the world quite fascinating. I don't know too much about GM, but have read about the effect of GM on lab animals. What I found most interesting is that crop yields are typically lower with GM foods vs conventional methods. Also the cost is higher, which is bankrupting some farmers.
2. Costing the Earth
GM crops are costing farmers and governments more money than they are making. In a 2003 report by the Soil Association the cost to the US economy of GM crops was estimated at around $12 billion since 1999, on account of inflated farm subsidies, loss of export orders and various seed recalls. A study in Iowa found that GM soyabeans required all the same costs as conventional farming but, because they produced lower yields, the farmers ended up making no profit at all. In India, an independent study found that BT cotton crops were costing farmers 10 per cent more than non-BT variants and bringing in 40 per cent lower profits. Between 2001 and 2005, more than 32,000 Indian farmers committed suicide, most as a result of mounting debts caused by inadequate crops.