I started out with a D70S and moved to my D300 shortly thereafter. It has been, and is, an excellent camera. Well built, solid, and with the MB-D10 very stable for shooting portraits and fast for HDR.
I've looked at the new cameras - and the D600 has excellent specs. I would love the higher MP's as I do a lot of cropping in post. But an old pro told me once that my camera is very important - and my lenses are absolutely vital. That's why I've spent the vast majority of my camera $$ on lenses - that will work with both DX and FX format. My most recent purchase is the 70-200 f2.8 VRII - and I paid more for it than I would have for a D600. And I made the right choice.
I will, very seriously, consider jumping on the D300 replacement when it comes out - but with the crop sensor of the D300 -- if I went to a D600 I would lose 100mm of reach with my 70-200. I'm not willing to do that right now.