Net detriment to society, obnoxious theatrics that accomplish nothing of real value, turns moderate people away from responsible and intelligent conservation.
They do raise awareness on some important issues, but they misinform on some key others IMO.
For example, I strongly disagree with their anti-nuclear campaign:
http://www.greenpeace.org/international/campaigns/nuclear/
My stepfather is a nuclear phycisist and works for the IAEA so I'm a bit biased, but probably more informed than most anti-nuclears