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Yep, I have used the D600, D700, and D800 quite extensively for wedding photography, which is quite possibly the ultimate test of low-light autofocus accuracy and reliability. The D600 definitely drops the ball much more noticeably than either the D700 or the D800, when light gets truly abysmal.
That, and the focus points aren't as spread out and the FX cameras all already have slightly restrictive coverage.
All in all, while I don't think it would be an issue at all for travel, landscape, or general photography, anyone who demands reliable accuracy in tough conditions will want to step up ton any camera with Nikon's FX 51 pt AF system...