I confess I have a gallon in my garage. I don’t know how to get rid of it. Haven’t used any for five years. Garden guru Steve Solomon mentions in his books that he occasionally uses Roundup and because of that endorsement I assumed it was okay. In fact, on Steve’s continuous newsgroup Roundup has been soundly debated. You can follow the discussion here.
All the latest studies indicate that Roundup, manufactured by Monsanto, is dangerous to human health and to the eco system at large. A summary of the evidence can be found at the Biosaf Information Centre or at Wikipedia.
Roundup is important to Monsanto in production of Genetically Modified Seed. As a result Roundup has entered the food chain with Roundup Ready Seed, plants that can be sprayed with Roundup and not affected.
On a more personal level, Roundup is dangerous for children and pets.
We don’t really need to use it. Boiling water, vinegar, a propane torch, rock salt, sea water and hoeing are all ways to control weeds if that’s something you feel compelled to do. And, lots of those weeds (dandelion, dock, butdock, etc.) are edible. No need to poison them. Eat them or juice them or compost them.