Karl,
I saw "Cowspiracy", and have to say my impression was much more favorable. I think the film constitutes a powerful incitement toward becoming vegan, and I would put it right up there with "Forks Over Knives", with this movie doing for the environmental arguments kind of what that one did for the health-based arguments.
The reluctance of mainstream environmental groups to talk about the impact of animal agriculture seemed to go beyond just not wanting to give interviews to the filmmakers. When representatives of some of these groups did appear on camera, they hemmed and hawed about it. Whether it's a true "conspiracy" or not I don't know, but it does seem to me that the environmental movement has tended to downplay, deliberately or otherwise, the amount of harm to the environment that could be avoided by switching to a plant-based diet and the importance of this step as something each of us can do to make a difference.
My personal recommendation is to go see it if you get a chance, or better yet send your non veg*n friends. Here's the listing from the BAVeg events calendar (http://www.bayareaveg.org/events.htm?EID=2785#2785):