I'm stuck in an annoying spot. The 5D Mark III is in every way that matters to me an improvement over my 1D Mark II but the 5D Mark III does not pack a sensor as capable as the D800. Other than the sensor though I prefer the 5D III and it is what I'm looking for. It just smarts to pay $500 extra for it which is one part of why I haven't yet (that and limited options for arca-swiss style L plates).
Image quality on the 5D3 is very good, yes, but the D800 has leapfrogged it in a couple ways. In most cases the differences aren't enormous but they're there. Of course back when I bought my 1D2 it was the other way around. I'm fairly confident we'll see the torch pass back and forth in the future but I don't really care to buy new cameras every few years, hence why I've been using the same one for almost 8 now.
I'd be a lot happier buying the 5D3 if it cost the same as the D800, but it doesn't. Hopefully the prices will drop before I actually make the jump. I'll grumble about how the D800s sensor is technically better but the 5D3 still beats my 1D2 in dynamic range, ISO, resolution, size, color, etc.
Too many lenses to jump, some which have no appropriate replacement on the Nikon side.