I have been engaged seriously in photography for some 52 years now -- that's 2704 weeks. The last 2 of those weeks have included life with my new D800. The D800 comes as the latest in a long family line of Nikons that includes the S Rangefinder and three lenses my Dad gave me. My conclusion at this point is that the D800 is simply a superb computer-driven picture taking machine. I think the biggest leap in capabilities I ever experienced was my transition from the D200 to the D700, but the D800 is certainly a substantial improvement over even the D700 body, of which I have two.
I would have taken the plunge sooner, but frankly, I was scared to. I watched with concern, the many posts here and elsewhere by people saying they had to use live view to achieve sharp focus, that they could only get great sharpness on tripods, and that diffraction was ever present in the back of their minds.
My conclusion is that these concerns are non-issues. There's no doubt that higher resolution, just as finer grain once did, will capture more detail, including mistakes. But I have found the D800 simply to be capable of capturing finer detail across a broader dynamic range than my D700 bodies were capable of. So I urge anyone who is considering a D800, especially going to one from a previous FX format camera, simply to drop the concerns and to go for it.
I think too many issues have been raised by people who spend much time photographing test charts and brick walls. Doing that with a D800 is like buying a Corvette and driving it only on a dynamometer-- an intellectual exercise at best. So pick up whatever camera you have, including getting a D800, and shoot photographs of people, places and things. Photography today is easier than ever. For those who complain about expense, when I moved from film to digital in 2003, a 36 exp. roll of film was running $10, plus processing. I just acquired two Lexar 32gb flash cards for $148 and can capture 720 highest resolution RAW NEFS and a basic JPG on the pair -- the equivalent of 20 rolls of film, not including processing!
At the age of 68, I can only recommend that we all enjoy photography--capture the world around you. And yes, the D800 will work just fine if you continue to employ your best technique -- even hand held.