Thanks Bernard I had previously read that and still leaning towards the Kirk but thought I'd ask if anyone had experience with the Ergorest product. Nobody that I know of had even heard of it.
I have the Ergorest MultPod, and like it--it works as a chestpod and tabletop (or cartop) tripod as well as a car-window mount; with a sturdy ballhead (I use the Acratech), it should it should work OK with your 500mm. However I use it only for long-exposure landscape & panoramic shots and the like. For wildlife, or candid photograpy in general, whether from a car or in the field, a VR or IS telephoto + a sturdy shoulder stock is infinitely handier and quicker to get in action, especially for fleeting targets of opportunity, and, properly used, will produce images to the limits of the resolution of the sensor (Nikon D2X) & lens (Nikon 200-400mm VR), as sharp as any tripod or car-window mount ever made.