I shoot a D700 and I am currently upgrading my glass. If you look at the life of glass, good glass is good for many years. It is a close to a one time investment as you can get. Good glass even holds it's resale value for years. Bodies on the other hand, cycle about every 2 years for DX and 4 years for FX as sensor technology and other features improve.
I have looked at the D600. For the near future I will continue to shoot with the D700 as it meets my needs.