I used to be a Greenpeace member however I also became a bit disillusioned in them. Their tactics don't always entail telling the truth, but however often are filled with plain lies and propaganda. Not that propaganda is bad, but it should at least be factual.
For example remember the Brent Spar catastrophe? Greenpeace claimed the thing was filled to the brim with toxins, turned out it was clean as a whistle.
And of course there's the whole seal issue in the eighties. Greenpeace verociously battled against the sealhunt. In the process the Greenlanders lost their livelihood even though there were still about 4 million individuals of the species of seal they hunted. 'But they're so cute!' Bah....
I've turned back to the WWF, of which I was already a member as a young boy =).