I purchased the D300 when it first came out. It was a significant upgrade from my first camera, the D70s.
I've enjoyed the D300 over the last few years, and have captured some wonderful moments of my young kids. With the D300, I had taken a few pictures that really stood out in terms of nailing exposure, color, sharpness, etc. Within the first dozen shots using the D600 I was already seeing photos that were simply better than what I could consistently get with the D300. Not better than what I could get with the D300, but than what I could get on a frequent basis.
Perhaps I never really learned the D300 the way I should have? I don't know. With the D600 I'm still manually setting exposure just like I do with the 300. The image quality is just better in my opinion.
I was waiting for the D300 replacement, mainly wanting better high ISO performance for indoor photography without flash.
The D600 came along, and I read and read and read. I didn't like the downgrade in FPS and lack of controls for some settings, but after the DXO test results came out online, I decided I would bite. Worst case, I use the D600 until the DXX comes out and then see if I want to sell the D600 and get the DXX. At that time, I won't loose very much money on the D600, and I will have gotten to use it to take more pictures of my kids.
I will say this after having the D600 for a week:
I'm blown away! I keep the ISO on auto at 3200, and I don't worry much about image quality when shooting in low light indoors anymore. There are still limits, but the bar has certainly been raised for me.
For my style of photography, the pluses of moving to the D600 from the D300 far outweigh the minuses. We'll see what the DXX brings......