Besides the rather nasty effect to people of giving them sunburns, the Giant Hogweed also is responsible for growing along riparian habitats, where it erodes and the soil. Also, since it can grow to such great heights, it quickly crowds out and eliminates native species of plants.
But there is even more bad news. According to an article in the Oregonian (as well as my own personal observation), this plant has made its way down to Oregon and is going to start spreading here quickly. It is already firmly entrenched in Washington. So if you are anywhere in the Pacific Northwest, and see what seems to be a Queen Anne's Lace on steroids, either don't mess around with it, or, if you have an NBC suit, kill it.