My biggest gripe with it is how the big-agri companies use really aggressive IP laws to intimidate farmers into using their crappy products. If you don't proactively buy licenses from Monsanto, they can (and probably will) sue you if your neighbor's transgenic crops cross-pollinates your non-GMO crops. The legal system practically treats it like it's 2002 and you're trying to Limewire yourself the latest album by Monsanto.

I seem to remember watching a documentary somewhere where they use their monopolistic control over the market to force exceedingly high prices on farmers in poorer countries too, forcing poverty wages.

If IP laws weren't so draconian and if the companies that made GMOs were held to a higher standard of ethical behavior then I don't see an inherent problem with GMO foods. GMOs are probably going to be crucial in surviving the next hundred years as a species with all of the unprecedented radical climate change we are going to experience. They're probably how we are going to colonize Mars and beyond.