My income tax money should arrrive any day now and I'm on the fence regarding a D600 purchase. I'm very, very happy with my D700 but would enjoy more mp's and video. I'd keep both if I do get one since my wife and 14 year old son use the D700 too although I think they would also enjoy the smaller lighter D600 body too!
I choose the D700 recently OVER the D600. My personal experience was not good - as the models I trialed were dust magnets, having visible dust spots after as little as 300 shots.
This is apparently not an uncommon issue and you can check out the d600 forums here for more info.
I was coming from a D7000 so the control layout of the D600 was very familiar to me, although I am finding the ergonomics of the d700 more to my liking. I really wanted to like the D600 as it has slightly better dynamic range than the D700 but I seriously couldn't fathom the continuous cleaning needed for a brand new camera.
I personally think the extra MPs can be as much a curse as a blessing as it will make camera shake more evident in photos. I am yet to encounter a photo where 12 MPs wasn't enough to get the job done. But I don't print bigger than 13x19. However, if the D600 didn't attract dust like a ceiling fan I wouldn't have been deterred by the extra MPs.
I have both a D700 and a D800E, love them both but I think I'm most comfortable with the D700, if I want to go shoot and have fun I take the D700, if I'm serious and shooting for a client its the D800E. The D800E just requires you to be on your game all of the time, it won't let you get sloppy. I am happy with my decision to own both, they are exceptional tools.
If I had a D700 (which I do) and wanted more camera (not right now), I would get a D800. I came VERY close to getting one.
It's interesting that the D7xxx and the D600 have the same basic controls approach, reminiscent to me, of my D70. The D300(s), D700/800 and D3/D4 all have similar top and back controls, different from the D7xxx and D600. Clearly two different approaches in Nikon-Land. Neither are wrong, but I prefer my cameras to be as similar as possible.
Wouldn't it have made more sense to make the D3/6/7/8xx family the same, and the D5/7xxx family the same?
I have a 600,700 and a d3s. I bought the 600 to play with the wifi and my iphone. I like the images coming from the 600 but I don't notice any difference in IQ from it and the 700. For me it's a mental exercise to embrace the new, to at least give it a try. shayne