The D300 is Nikon's premium DX format camera, offering professionals and extremely serious amateurs a full-sized, high-speed, professional camera when fitted with the vertical grip, or a comparatively small and lightweight but well specified workhorse when used without.
A lot of people are saying that this camera is the real replacement for the D2X. This is both true and not true. In terms of resolution, frames per second and battery life (with the vertical grip), it matches or exceeds the D2X. However, it lacks the robustness and waterproofing of its larger cousin. If you can't see the point of robustness and waterproofing, then the D300 is pretty definitely the best DX format camera you can get. If you can see the point of them, a second hand D2X, or moving up to the D3 is probably the best way to go.
The D300's specifications, however, only tell half of the story. Nikon has accomplished a near miracle by cramming the resolution and frames per second into this camera. But the real revolution is in the electronics. In common with the D3, the D300 corrects for chromatic aberration in camera, which means that virtually every shot on most lenses is sharper and crisper. The D300 also has better (51 point) autofocus, metering and, crucially, white balance. Other electronic benefits include Live View, which gives dSLR users the best of both worlds -- magnifiable rear LCD live view, as well as true through the lens SLR view (but not both at the same time). The D300 also includes DLighting (a kind of tone mapping) and the ability to apply some basic alterations in camera after shooting but before transfer to Photoshop. How important these are depends on what and why you are shooting, but it may mean that you can hand a client a set of production quality JPEGs straight after the shoot, or send them to the PictureDesk, without having to use a computer.
Finally, for those who haven't yet mastered sensor cleaning, the D300 has a self-cleaning sensor.
What's the verdict on the D300, and who should buy it? At half the price of my D100 just six years ago, and one third of the price of my D2x just two years ago, this camera is amazing value. The price is slightly higher than it appears, because to get D2x beating frame rates you have to have the vertical grip, and have to get the D2x style battery to go with it. But this is not a significant extra expense.
If you are a professional and you like the huge, chunky feel and weight of the D1, D2 or D3 series, then the D300 feels a little flimsy and insecure -- even with the vertical grip. On the other hand, if you prefer something which won't worry your physiotherapist, the D300 is a great camera. Nikon categorises it as 'professional', which means ownership of a D300 gives you access to Nikon Professional Users, and ownership of the D300 and one other pro camera, along with the ability to demonstrate you earn your living from photography gives you access to Nikon Professional Service, with all the attendant benefits. You can, of course, opt for the D3, with its extraordinary low-light performance, but the D300 is itself no slouch, and its resolution is big enough to shoot a bill-board without upscaling.
If you are an amateur, and you have the money, then the D300 gives you access to the finest DX format camera that Nikon has ever produced without the enormous weight of the D2x. The resolution is sufficient to produce fine prints at up to 30" across, with judicious post-processing, and the noise performance is better than any other Nikon DX camera. The built-in chromatic aberration control means that the D300 produces genuinely sharper images than the D200, D2x, D80 or D40x, an advantage that the D200 did not have over, say, the D40x.
Nikon surprised a lot of people when they released this camera, as many believed that they would bet their future on the FX format for professionals. The actual performance of this camera for its weight and price - in my mind - amply justifies Nikon's decision to continue to innovate in this format.
Warmly recommended.