A couple of things to add to the earlier thread after using the combination. Note that I have the earlier non-VR model of the Fieldscope. The Fieldscope is great and the 20x-60x zoom eyepiece is very bright and clear. Image viewing is very good out to 45x and then a little soft beyond that point. 60x is a real stretch.
As Neil suggested, image quality from the scope has a hard time matching a 600mm lens and teleconverter with cropping. The Fieldscope is slightly soft at long focal lengths. That means I end up using the scope/adapter at 20x-35x for sharpest images. Beyond that softness and loss of light makes it hard to get good images. The image captures are great for identification and proof, but won't make it as as large art prints. Some birders have a life list that requires a photo and this is a good approach.
The scope is manual focus only - which takes some practice. With the D300 or a DSLR there is a focus indicator light that indicates which direction to adjust and whether you have achieved focus. Birds in flight is possible but takes lots of practice.
As far as setup is concerned, I really like the large gimbal head, but its a bit of overkill for birding. In reality you could get away with a lighter capacity gimbal head if you are not also using a big lens.
If your primary objective is birding with digiscoping the Fieldscope and adapter will be a good combination with your D300.
Communication between the Fieldscope and camera is not quite as good as I would like. Aperture is reported at f/13 and the actual focal length used is not reported. But It does indicate manual focus.
Attached are a couple of images with the D7000 and Nikon EDG 85 Fieldscope with Nikon EDG FSA L-2 DSLR attachment. This will give you a pretty good idea of image quality at 30-35x zoom. These are sandhill cranes with the Asian Hooded Crane sighted in TN early last year.