It's quite a jump up from D100!
Almost worth it for the LCD alone. I can now tell when I've taken good/bad pics.
I bought the D700 for low light (small-time bands in dingy bars,moshpit, no flash).
Shockingly good to ISO6400. I've only used the flash 3 times on people (at their insistance).
"Did you get the picture?" is a often-asked question.
I leave it in Manual with and let AUTO ISO float up to 6400, no worries.
As a side-light to the low light capability, it really rescues old mirror lenses (fixed at F8), since I can take pics at 1/1000+ in most reasonable light.
Since all mirror lenses for nikon are manual, the manual focus aids (the triangles near the focus dot) help tremendously.
My eyeglass prescription is odd enough that I can't reliably focus by eye, so even with the thin depth of field, I can get sharp pics.
The fact that it is full frame makes it loads of fun with wide lenses (18mm).
For me the full frame is just an added bonus.
The fantastic colors and dynamic range are something I wasn't planning on, but sunsets just pop.
My only "complaint" is it's heft - not bad, but I gotta have a "con" with all the flowery "pro"s.
I'm so glad I didn't wait for this sensor/camera in a smaller body. Having too much fun.
Bought refurbished, added ~4000pics in just a few months.