Hi guys, this is my first post here (took a while to find a topic I thought I could contribute to)...
I upgraded from a D90 to a D700 and it was absolutely the best thing I did. It's not about the IQ (cos lets be honest, no one makes really bad bodies anymore), its all about the feel. The D700 feels awesome. Its beautifully balanced (especially with a pro lens) and sticks into your hand like glue. Like a previous poster said, it has encouraged me to take more pictures than what I did with my D90 and this has made me a better photographer. It gets out of the way of taking photos.
In terms of weight, I just lugged the camera around Banff and Vancouver with the 24-70 and a 50 1.4 and it was not too heavy at all (I use a sunsniper strap). Not only that, the quality of the pictures taken was pretty good (even by my standards). Would a D90 with the 24-70 or 50 been any different in terms of IQ? I doubt it - but would I have wanted to take as many pictures or felt as good taking them in -19 degree conditions? I don't think so.
Another thing about the D700 is that once you get used to it, it allows you to do everything you need without your eye leaving the viewfinder. You see differently with it. Also, in difficult conditions (cold, rain, direct sun, etc...) it is good knowing that this camera is over-engineered for what you need it for. Always turns on, battery life is superb and it is rugged and reliable.
Was it expensive? Absolutely! Was it worth it? Yep - for the reasons above and many more.
Mark.