You get GMO plants. And technically, you can't save seeds from those plants because the company that designed them owns the patent and doesn't give you "permission" to use their genes. Which is why Monsanto spends time going after farmers who try to save seeds...they're supposed to buy new seed every year from Monsanto. Even farmers who plant non-GMO corn can have their corn crop contaminated by pollen from the neighboring farm's GMO corn (since corn is wind pollinated) and then Monsanto can actually even go after them and if the seed that they saved is found to contain Monsanto genetics in it, well, let's just say it's not pretty.
As to what is bad about GMO seeds, one concern is that we don't know the long term effects of the consumption of genetically modified foods. And with the Round-Up Ready seeds, we then are certainly increasing our exposure to herbicides (since they now can be sprayed on the crops without worry that the crops themselves will suffer). Also, we don't know if herbicide resistant crops might not cross pollinate with weeds, creating herbicide resistant weeds. Some corn has been engineered to contain pesticides to protect it from corn earworms, but it might also adversely affect not target species, such as monarch butterflies, or if this might lead to pests being resistant to the pesticides. There is just so much that we don't know about the long term safety of GMO crops.
Interesting article here:
http://www.csa.com/discoveryguides/gmfood/overview.php One of the things that I found very interesting in that article was a brief discussion of the different regulatory agencies in the US that are charged with determining the safety of GMO foods and how the whole thing doesn't seem to be very effective:
In the United States, the regulatory process is confused because there are three different government agencies that have jurisdiction over GM foods. To put it very simply, the EPA evaluates GM plants for environmental safety, the USDA evaluates whether the plant is safe to grow, and the FDA evaluates whether the plant is safe to eat. The EPA is responsible for regulating substances such as pesticides or toxins that may cause harm to the environment. GM crops such as B.t. pesticide-laced corn or herbicide-tolerant crops but not foods modified for their nutritional value fall under the purview of the EPA. The USDA is responsible for GM crops that do not fall under the umbrella of the EPA such as drought-tolerant or disease-tolerant crops, crops grown for animal feeds, or whole fruits, vegetables and grains for human consumption. The FDA historically has been concerned with pharmaceuticals, cosmetics and food products and additives, not whole foods. Under current guidelines, a genetically-modified ear of corn sold at a produce stand is not regulated by the FDA because it is a whole food, but a box of cornflakes is regulated because it is a food product. The FDA's stance is that GM foods are substantially equivalent to unmodified, "natural" foods, and therefore not subject to FDA regulation.