Not to trash the D600. . . No, really . . .(Yes, I would take one for the right price and situation. --Even considered selling the D700s to get 2 D600s. Then I read the feature set.) I DO remember having my new D100 with 1/180 flash sinc after having 1/250 since the N8008. Being used to 1/250 (which wasn't--and isn't--enough many times shooting anything moving at a brisk clip), the D100 was a pain with flash and helped me have a lot of blurred flash shots. I really didn't need any help. I was good at it on my own. I had developed that skill on my Nikkormat with 1/125 years before. I was SO happy to get 1/250 back with the D200!
I have considered the D600 as a back-up, and might get one down the road. I'm sure the extra pixels and newer processor are great improvements, but sometimes it's more than just highest resolution. For those great improvements and slightly lower price point, you're giving up some other things in this particular body--just as you do in every decision like this. Every body decision is a trade-off unless you have unlimited resources. If you want it all, I guess you will have to pony-it-up for the big-dog bodies or be a little more careful using flash. It can be done. It's just not a lot of fun.