>thanks for posting the question .. I am going thru the same >dilemma and have been following thru the threads .. I >currently have a D800 and have a D4 on order .. >Marv ..
Well, I did it, and had my first outing last night at a rodeo. The jury is out on many aspects, but the speed is amazing. Roping or a bucking horse at 11fps gives you a lot of choices of expression and position. Unlike (say) volleyball or baseball where at least in theory one can hit the moment of greatest action with good timing, this is more random, and it is a huge benefit in comparison.
However, I've been spoiled by the resolution on the D800. I've gotten in the habit of framing loosely to make sure I get everything, and with the D4 (and shooting at 6400) I'm finding myself saying "why didn't I zoom on more" when I look closely and faces are filled with noise.
I shot with both, swapping lenses back and forth, and then loaded all in lightoom and started culling (not noticing which was which or having it displayed). When I was done I had about 3 times the initial keepers with the D4. That's a bit unfair, as I also had probably 5 times the images.
But it was very nice having two bodies. While bright I a long lens on the D800 for action on the other side of the arena, then would switch to the 70-200 on the D4. Awkward to carry two, but SO much easier.
But yes... 10fps is just a whole different world from 4fps.