on one of the local wedding forums I visit, a nikon user was asking for some upgrade advice and this is what was said, which I find quite interesting and relavant to the whole 5d3 vs d800 debate:
" I've spent around 2-weeks exhaustively researching my next camera. I do not think that the D800 is good for weddings... it is extremely slow shooting and you are paying for a lot of resolution that serves minimal practical purpose in retail photography (i.e., a 12mp cropped file makes a gorgeous 24x36" canvas).
If you can handle a single card slot, I think the D700 is presently the best value camera in Nikon's lineup for weddings. I owned one and have used many others and never really had an issue with it.
I think the D3s is the best wedding camera on the market today. Fast shooting, exceptional AF, amazing high ISO capabilities and ample resolution... even in 14-bit uncompressed RAW, I have never hit the buffer (and I am one fast shooter)!
When I shoot events, I use a D3s as my primary camera and a D3 as my backup and feel very comfortable with that set up."
"As an owner of the D800, it never comes out at weddings, I stick to the D3s. As Brady said, it is just far too slow of a shooter. Plus, the files sizes are too much of a hassle to drag around and edit. A 16 bit one layer tiff is 289.2 mb per file. I would be looking at a D700, D3 or D3s."
"The D700 is amazing! All the features of the D3 that meant anything and was about $2k less. I love that camera. I still use mine today and it has been three years. The D800 is overkill for weddings. It should come out for portraits, maybe some details and that is it. It is total overkill for the rest of a wedding..."
There ya go, for all the rave reviews, there are many who are opting out of the d800. Grass is always greener!