If I may humbly add my opinion to this list of venerably more knowledgeable contributors here, the D800 is the first DSLR camera that I am completely comfortable using with
enabled. With previous cameras, I was always concerned with noise at high ISOs; with the D800 I am confident of useable, clear images in situations where with previous cameras I might have had to use a larger aperature or slower shutter speed than I might have desired for the image I was trying to capture. The ISO capabilities of the D800 give me the freedom to manually adjust the image as I see fit without the worry of excessive- or even noticeable- noise at higher ISOs. I hope this helps.