My D800 worked fine right out of the box. I ordered it in February and got it mid-July. After about a month of shooting (I had D800 withdrawal if I didn't take photographs with it every day), there was a crack in the sensor - I never touched the sensor or lifted the mirror to look at the sensor. Nikon replaced the sensor on warranty and appeared to have checked the AF as well. I had been experimenting with AF C 3D when I sent my camera to NY, and it was in AF S single point when I got it back. There was also a small issue I had with the memory card door, which they also fixed, and my D800 was back in my hands two weeks from the day I sent it. For me, my experience with Nikon repair was great.
I strongly suggest that you also get one of the books on the D800 if you buy the camera. I have Darrell Young's "Mastering the Nikon D800" and find it indispensable. This camera has lots and lots of features and options, AND the manual don't give a clue when or if to use them.
I agree with Roger whole hardly that the D800 requires practice, patience and experimenting with the settings to find what works best for you and your style of shooting. Careful focusing techniques with the D800 will be rewarded with fantastic photos.
I love my D800 and I've always been thankful that I bought it!