The full repurcussions of GMO's have yet to be realized. However, there are some very specific problems associated with them. Firstly, the pesticides and insecticides used that the GMO's are specifically designed to be immune or uneffected by, are applied more rigorously. This means more chemicals on your food. More poison on your food is bad. There are even cases where these chemicals have produced allergic reactions in children. These two reasons alone don't consider the biological impact that may happen through the generations. How much evidence is needed?
I do not think that there are any bad effects from genetically modified food. While the science and process is still pretty new, there still isn't really any concrete proof that the people that eat genetically modified foods will get bad effects. I think that until then, it is safe to assume they don't.
Science has shown that genetically modified food poses no greater risk to human health than unmodified food. News articles and other organizations are promoting fear mongering of the public over this issue with no real proof at all that GMO's are harmful to us. Without genetically modified food we would have trouble feeding the population we have, as GMO's produce a higher crop yield than unmodified versions, and are more resilient and pest resistant. The only downside to GMO's is the main company that produces them almost has a monopoly. GMO's should be open source.
From what I have learned about genetically modified food, there aren't any bad effects. They have the same nutritional value and can be grown larger and more quickly. If anything, from what I have gathered, genetically modified foods are a boon and not an issue, so they should be embraced.