I guess it might depend on whether it'd be your only lens long-term, or if you'd have (or plan to get sometime) better glass available for other uses. Or it might depend on how good you are at working within a lens' strengths and weaknesses.
Boy that may have sounded a bit snooty; sorry about that. I'm just a person that loves to take photos - by no stretch of the imagination can I be considered anything other than an amateur.
I currently have an 18-200 on my D70, and enjoyed using it quite a lot. I did have a couple other lenses, though, that I used when necessary - such as a Nikkor 35mm f/2. But I've found that (in my case), on a given outing, I'm rarely shooting "all over the place" in terms of focal length. On a given day I end up shooting mostly wide, or I might shoot mostly long. Given the inherent quality trade-offs of a superzoom, I've decided not to go that route with the D700. But I'm not immediately buying pro glass (can't afford it right now, for one thing). I'm just starting out with the "best bang for the buck" used lenses I can find, like the 24-85 f/3.5-4.5 AF-S. Then we'll see what Nikon's offering in a year or so, since I expect they'll have to update a lot of their full-frame glass. I'll also have a better idea of how I like to use the D700 at that point.
In any case: Despite what anyone says, in the end all that should really matter is what makes you happy. They're your photos, after all. Some may think a superzoom is indeed a waste on a D700; but it's YOUR D700, so if you are enjoying using the superzoom then it's not a waste at all.