Round-up, the most widely used herbicide in the world, is nothing but bad news. It is not a long term option for agricultural use, and if you have it you should throw it away, stop being so lazy, and pick those weeds out with some gardening tools or your hands instead. Although the herbicide portion of Round-up has been extensively tested in the labs, the emulsificant that binds it to vegetation has not, and its effects on human health and the environment are yet to be well documented (What happens when it gets carried away as runoff and is absorbed by water, soil, and living organisms?). But lets look at what we do know.
Nature finds a way to overcome challenges whether it be from predation or some other environmental pressure, in this case poison. Most plants sprayed by Round-up will die, but given enough time a plant resistant to Round-up will emerge, crafted to survive and to distribute its code, effectively circumventing this particular poison and thriving in the lack of competition.
Screwing with a crop's genes to make it resistant to the poison gives the crop the unnatural advantage of already having the right traits to survive, but this advantage is finite and will eventually cease to be an advantage when the other plants evolve. It becomes quite clear, that much like any other Green Revolution technology, this is a short-term strategy (Green Revolution agricultural practices are inherently unsustainable because they mostly depend on petrochemicals to develop chemical fertilizers, pesticides, herbicides, other poisons, and also to power agricultural machinery. Yes, petroleum is a finite source, unless you don't mind for solar energy to be stored in biomass and then for that to be acted on by geological pressure and other conditions for a very, very long time. When the oil dries up, new solutions or materials must be found in order to continue which will most likely signal the next agricultural revolution, or at least a major innovation).
Alarmingly, the possibility of genetic leakage into the environment has not been sufficiently addressed. To what extent do engineered species' genes spread throughout the native population, and what implications does this have for the environment and us? No one really knows, and the big corporations like Monsanto have no interest in addressing this because it would be an added cost and likely yield results that would advocate a more cautious approach to genetic engineering. Well, it was only a matter of time before such recklessness bit us in the ass. Check out this story about Round-up resistant coca plants. Is this the first clear example of genetic engineering being used by rogue scientists, or is it merely the result of natural selection?
If that got your attention, why not read about the Terminator technology(and here) that Monsanto has developed. Or about stealing the genetic heritage (this is, interestingly an IPR issue) of farmers that don't know any better or don't have the resources to do anything about it(scroll down to no. 4). The bottom line is that Monsanto's products and services are designed and sold in order to maximize profits, as are most successful businesses. I'm not saying that seeking to maximize profits is a bad thing, but when the interests of an elite few compete with health, social, and environmental well-being of everyone else it's time to start paying close attention.