Switched about 18 months ago. I miss the convenience of the D300 with 18-200. I don't like the weight of the D700 with its heavier glass. BUT, Files are smoother and cleaner and can tolerate an amazing amount of postprocessing, I regularly see up to 2 stops highlight headroom that I can recover. Also you don't have the D300 grainyness in the sky on at the lower ISOs.
I was frustrated in the start due to the step down in convenience. But the D700 grows on you. There are just so many cases where it absolutely "nails it" in terms of focus, sharpness, lighting etc. The extra high ISO performance is a plus, and no amount of VR in the wider DX lens will help stop the action
Finally, not having the 18-200 "all in one lens" means that I had to back off and learn to use compose using the 24-70. That has been a good discipline, and has led to a lot more interesting compositions. I discovered that with long reach zoom, I ended up zooming way to often, getting un-interesting shots.
Previously I had always suffered from "upgrade-itis", moving quickly from one model to the next (D70, D200, D300). Now I'm happy, and am not even tempted by the prospect of the D3s in a D700 house.
Carsten