The dust issue isn't a killer - just clean the sensor like you did with every other camera you had before this generation of dust shakers.
Is the D600 great as a full frame camera? Yes, especially at the price. I personally preferred the 24-120 VR II vs the 24-85 VR kit but the kit lens is no slouch either. It just wasn't for me.
Of course, it also depends on what you shoot too. If it's fast action and low light then you might, just might, be better served by a used D3 or D700. Megapixels be damned!
My only gripe with the D600 is actually the button layout compared to other Nikon bodies. They moved some functionality around like the OK button zoom feature that is on all Nikon pro digital bodies and instead it brings up the retouch menu. I'm sure that this is deliberate to differentiate between the D600 and it's bigger brothers. Ditto the highest 1/4000 shutter speed. Arguably the heritage is from the D7000 and not the D800.