Roundup is very effective as a weed and grass killer. It comes with a convenient applicator that makes it possible to not have to come into contact with the poison itself, which is an advantage. I have used Roundup before, and it works very quickly and effectively to remove weeds from unwanted places.
I am an environmental scientist and study the effects of pesticides on amphibians. Recently many studies looking at the effects of Roundup herbicides on frogs have found that this substance is very toxic, and can kill nearly all the tadpoles (frog babies) in a pond. Although Roundup is not made to kill aquatic weeds and so is not applied in water, this pesticide can still find its way into water via runoff or overspray.
So, if you are needing to kill weeds quickly and effectively and have little or no way to introduce this herbicide into a body of water then Roundup is definitely an option. It can be relatively safe environmentally if it is applied according to directions and away from wetlands such as lakes and ponds. If you are looking for a weed-kiling option that is a little more frog-friendly then Roundup is not a good choice.