Hey photo buffs.
Just before Christmas I upgraded my kit from the Nikon D800 (which is still around serving my Macro photo needs) to the Nikon D5 as my primary platform. I'm providing a review below as there weren't a ton available when I was looking.
Out of the Box
The D5 comes out of the box as a behemoth. The weight is hefty and particularly coming from the lighter D800 and Df platforms its presence is noticeable. It is solidly built and the majority of the casing is wrapped in the rubberized texture grip that Nikon is favoring for most of its models now.
Aside from the heft, there's just about every port imaginable on the camera, for all kinds of auxiliary toys and lights as well as the hotshoe flash and dual XQD (as opposed to the dual-CF alternative) slots.
Nice changes in this platform for me are the button placement. The ISO and Mode button switch places, which means you can cycle ISO with one hand now since it's right next to the trigger. Since real-time ISO changes are far more common that needing to switch from Manual to Aperture mode in the same shooting block, this makes far more sense.
The screen is what many Nikon-ites have been raving about, finally introducing a touch-screen. Being a bit of a dinosaur when it comes to touch screen devices (I still use a blackberry if that tells you anything), this isn't a selling feature for me but some of my clients have appreciated it. The ability to quickly zoom in and out with the pinch gesture and use the image scroll bar to go through a long list of burst images quickly is handy, but definitely not worth the price of admission if that's the only thing you're interested in.
Last comment, the addition of the LED settings bar at the bottom of the camera's rear, below the preview screen, is a nice touch.
Don't buy the D5 if you're into architecture or landscape photography as your primary use. Once this baby goes on a tripod it's selling features drop significantly in value. I found the D800's performance for landscape, architecture, and astrophotography as good if not better than the D5, particularly since having twice as many pixels meant more ability to crop and do large sized print blow-ups with more forgiveness. The D5 handles the images just fine, but there isn't enough noticeable difference to justify the cost.
(Seven Sacred Pools - O'he'o Gulch, Maui)
Wildlife is exceptional with the D5. Birds, whales, and other animals I shoot don't like to stay still for very long, and so they're being shot on very fast exposures. The D5's ISO range is impressive, and the image sensor is a marked improvement over the D4, giving the same level of grain at about double the ISO setting as the other cameras in my kit. (So an image shot at ISO 1600 on the D5 looks like an image shot at ISO 800 on the D800). This means more ability to shoot crisp images in challenging lighting conditions. The second fact that can't be understated is the autofocus. I measured the autofocus on the D5 to be 3-4x faster than the D800 using the same lenses, and 2-3x faster than the D4. When dealing with unpredictable, fast-moving targets like a breaching whale or running deer, this is critically important. Definitely worth the upgrade here.
(Honu @ Ho'Okipa Beach Park)
Sports is pretty similar in the review to Wildlife. I find the D5's second auto-focus sensor is a game changer. With the D800 and when I shot with a D4 previously, you get issues in sports with players moving at high rates of speed. When a hockey player is racing towards you, autofocus locks in at one point in their skate and struggles to refocus for each subsequent exposure. In high-burst mode, I'd get 1-2 frames in focus with the D800, 3-4 frames with the D4, but with the D5 I'm getting a whopping 9-10 frames in crisp detail, even as I am adjusting variable zoom at the same time. This is incredible. Being able to catch the race for the puck and the goal in the same burst with the same lens makes this upgrade worthwhile.
(Lethbridge Hurricanes vs. Kamloops Blazers - Canes Captain Tyler Wong trips over the opposing goalie and get airborne)
I have a pretty aggressive position on equipment for portrait photography. If this is all you are doing as a photographer, there's not a lot of value in going above entry level DSLR. You want a minimum of 14MP to be able to "blow up" images to wall print size (2'x3' or larger) but beyond that, you're far better to invest in high quality lighting equipment or a really solid prime lens than to spend the money on a camera. In portraiture your ISO will rarely go above 800 and your shutter speed is usually 1/320 or less, so the ability of higher grade equipment to handle higher values in there categories doesn't provide much benefit.
So, like most decisions in photography, the question is what you're planning to use your gear for. High-paced, fast moving subject matter like sports, wildlife, or more staged shots. The D5 fills a big hole in terms of autofocus speed and low-light performance, but beyond those upgrades, has little value if you're shooting more stable content in controlled lighting conditions.