I've been considering a camera body upgrade for a while now, as the limited dynamic range of the D90 (and its poor low light performance) are beginning to limit what I can get done out and about. Towards the end of last year, I went down to Wex in Norwich to have a hands-on with some potential candidates. I can't recommend doing this highly enough - some of my views and (mis)conceptions about certain bodies going in were shot down as soon as I got the cameras in the hand.
The image above was taken by a Nikon D810, of a Nikon D800. Although it was a big body, as soon as I got the D810 in the hand it felt amazing - solid construction, perfectly laid out buttons, and a big bright viewfinder. But the two shining features to me were the grip shape (sooo comfortable) and the feel of the shutter. It had such a satisfying clunk to it! In terms of the image, beautiful bokeh and colours straight out the camera (note - the image above has had some minor editing to reflect real-world use of the images). If this was the camera for me, I would need to remember two things - depth of field is crazy-shallow with the full frame sensor; and two, because of the ridiculous number of MP I would have to make damn sure my technique was spot on to produce clean, blur free images.
This is an image from the (slightly older but still awesome) D800, to show the amount of detail you can get from this camera. Now, it should be pointed out that I was not using the best lens while taking these test images (I just took whatever they were offering), but even so the level of detail is amazing. When viewed in Lightroom at 1:1, every bump on the body cap can be seen. For landscapes, this would just be incredible (especially coupled with the fantastic dynamic range - discussed by Dom Bower in this vid: https://www.youtube.com/watch?v=-WRt32pRmLw).
I just threw this image in at the end just to show an uncorrected image from the D800 (it has been sharpened slightly, but everything else is as it comes from the camera). Lovely, vibrant colours, again with excellent sharpness and detail.
Overall, I was really impressed with both bodies. Obviously, the D810 is newer, has the delicious-sounding new shutter and extremely comfortable grip. However, the D800 is still an amazing camera and has the added advantage of being cheaper (through the second-hand route). Either camera would be an amazing purchase, and considering the latest output issues with Nikon (flare, dust, oil...) these are both excellent options for any landscape (or indeed portrait) photographer. All I need to do now is decide which one...