I've been saving for a large prime lens for a few months now. I'm torn between the Nikkor 400mm f2.8, the 500mm f4 or the 600mm f4. All are fabulous lenses, each with its own pluses (and a few minuses).
To help me make up my mind, I occasionally rent either the 400mm f2.8 or the 600mm F4 from Pro Photo Supplies in Portland. They have a nice weekend deal.
The weekend of April 20 (how time flies) I rented the 600mm and spent some time at Ridgefield National Wildlife Refuge. In addition to the lens, I rented a 1.7x teleconverter which upped it to 1000mm. Multiply that by the 1.5 increase gained by my D90's smaller APS-C image sensor and I was packing a 1500mm telescope.
It's really amazing to look through the view finder and see, with such clarity, birds that are quite far off in the distant. But it requires a lot of care to keep the camera steady. The least movement and your picture is ruined.
The image of the male Cinnamon Teal above was taken in the canal between North and South Quigley Lakes. This is a full frame shot (no cropping). While the ability to get in this close is amazing, it can be hard to fit birds (especially larger ones) into a frame if they are too close.