GM is both good and bad.
we wouldn't have wheat and many other modern foods without a form of GM, although this selection and breeding takes many years and is by trial and error mostly. all modern food stuffs are GM'ed in a way as most farm seeds are F1 hybrids. GM as in what grabs the headlines is selective breeding in a very short space of time rather than over many years and often involves splicing genes from different plants together. Pest resistance can be a good thing but the as in nature, cross breeding/pollination does occur naturally so the worry is that pest resistance will spread to plants that we do not want it too. this is more essential in herbicide resistance. a plant is spliced with genes that make it resistant to herbicides so that fields can be sprayed to kill off all the weeds but not the crop. as weeds and crops grow in close proximity the worry is that the herbicie reisistance will spread to some weeds and then you are back to square on. all pretty harmless in the scheme of things but as no long term studies have been concluded yet, it is simply not known what messing around with crossing genes will have on the environment and us, who eat the end product. some early forms of GM crops actually raised the likelyhood of stomach cancer, others increase the risk of asthma in preliminary studies.
natural genetic engineering can be very good though, take the rice studies in the phillipines. strains of rice have been developed over many years without gene splicing that have increases rice yeilds by 400% and new strains are being developed all the time to suit different environments. rice can now be grown in places that 50 years ago you wouldn't dream of doing so. wheat several hundred years ago grew 6ft tall and had 1 or 2 ears (1 or 2 lines of seeds on the stalk at the top), now you have 4 eared wheat that grows less than 3' tall saving the plant lots of energy and growing twice or more wheat per acre. (the scene in gladiator always gets me, when he is walking through the field of wheat towards his house he runs his hands across the top of the wheat that clearly has seeds on it and is ripe, yet in spain 2000 years ago, the wheat would have looked nothing like that as they firstly grew spelt and it would have been taller than him when ripe!!)
GM'ing mainly increases reliance on big companies for seed production. most crops will be sterile so you cannot collect seeds for next year so have to buy more seeds from the big bio companies, so although you may get better crops, you pay through the nose for them and the accompanying chemicals needed for many of these crops and many farmers in the countries that could do with these increased yeilding seeds cannot afford them or if they do manage to do so, the end price in local markets is so high they can only afford to sell abroad, benefitting western markets but failing to feed the people who really need it. (western countries can then feel good about themselves by then shipping food aid to the poorer countries in need of food, yet again increasing the reliance on western aid and goods).
after reading this you may think I am anti GM? I am not, but GM'ing by splicing genes is dangerous when introduced into the wider environment without many years of testing. natural selection to increase crop yeilds has worked and worked well, cross breeding widely differing plants with gene therapy has the potential to be disasterous. At uni, I read about a bio companies plan to cross japanese knotweed with a cereal crop, luckily that was stopped due to its status in many western countries.
personally I would only allow natural genetic modifications, the type that has gone on for thousands of years. if 2 plants that are naturally cross bred do not fit together, the plant doesn't grow, but by splicing genes together you can get things to grow without knowing consequences. GM is about money and dominance by the big bio companies, it is not really about feeding the poor and hungry.
(sorry, will climb down from my high horse now )