I have too. I had the D7000 for 9 months after using the beloved K5 from Pentax.
I used the camera with Sigma 30mm and Nikon 85mm 1.8 lenses. As far as sharpness, colors and AF, the D7000 was a killer for me. The colors seemed a little washed off after coming from Pentax, you can get stumping colors with Pentax RAW's if you know your way around PP. Well it was a learning process and I learned to get very nice, if not stumping, colors with the D7000 too. With Nikon you just have to PP a little finely to get very good colors. I didn't like the SOOC colors in D7000.
The want for the D700 was purely due to 2 reasons; shallower DOF and a more rugged body. The D7000 felt a little flimsy in my hand, and the grip was a little too much thin for my taste. The camera was like a killer technology wrapped in a flimsy shell. It just did not make you feel the sturdiness of a K5.
So I made the jump to the D700. At first it was not a lot of difference. You need to learn your way around the D700 too. The differences were; you could feel that you took a step back in terms of technology. Jpegs of the D700 never really amazed me. But I generally shoot RAW so that was not an issue. The WB seemed a tad on the cold-blue side too. But that was also easy to compensate, so no worries there either. The shutter is much louder compared to the dampened and fast D7000 shutter sound, which I enjoyed very much. Shutter sound can be overlooked too, but it is nice to know that you can reduce that sound even more in times of need, not to startle people, etc. Comes good in street candids. But the D700 shutter sound is attractive in its own way.
As to the positive sides of the jump, yes there are killer ones. Firstly, the AF system is way more accurate and comfortable to use. Crosstype sensors of the D7000 were doing a fine job for me, I didn't have any of the D7000 AF problems largely mentioned in the past and I was happy with them in lowlight shooting too. I could live with the little hunting the camera made, but I am fairly good at MF too, so D7000 AF was not a dealbreaker for me. It is worth noting that I got great lowlight AF results with the Sigma 30mm. But the D700 has an AF system that just does its job wherever you point the camera. It was much more decisive when pulling focus to low contrast areas and in lowlight, rarely hunted (i.e. with an 50mm 1.8 D). It was more enjoyable to use D lenses since the camera motor has a higher torque. While not instantaneous, the D lenses focused very quickly and accurately too.
As to metering, the D700 is much better too. The D7000 had a tendency to overexpose when there were skin tones in the scene, I read it in a review and experienced in first hand too. Also it tended to overexpose a little when there were pronounced shadow areas in the image too, although not in a way that may lead to unwanted white clipping in the D90. The D7000 used the logic of ETTR I guess. But I did not get along with those features well since I could always correct shadows or skin tones in PP. The D700 always finds the balance in such situations. It just correctly exposes. I do not remember using any constant EV compensation on D700 any time.
I was able to get smoother colors as well. In PP it seemed like the images had richer colors and color transitions. Coupled with the magical "pop" the images had, it was a very nice experience to take photos with and process RAWs of the D700. The images seemed a bit more 3d. You can get it with the D7000 too but it is easier with the D700.
With all that said, I don't know it was a problem with my D700 sensor or my lenses, but I found the shadow noise a little harsher than the D7000 and K5 raws. I was comfortably lifting shadows and exposure in those cameras with very little loss, especially in the K5, but the D700 shadows seemed a little more fragile. I even saw shadow banding at times. I don't know what to attribute this situation to. Maybe it was my lenses' inability to gather much detail. I could not buy high-end lenses for my D700 since I had to sell it after 4 months due to financial difficulties.
Other things that changed with the D700 were;
LV use. The D700 did not provide a smooth LV use, it was laggy and not practical when you zoomed in (for macro, precise focusing, etc). If you use LV much, that may result in loss of practicality. That's the result of historical technology.
Image playback. You have the option of one-touch 100% zoom in the D700 at the selected AF point. Which comes in handy in checking sharpness of a shot you just took.
The ability to use wider FOV with shallower DOF. It's very enjoyable and what the FF brings to you. I also used a couple of legacy MF lenses with m42 adapters, which lenses were too long on the D7000.
Bigger viewfinder. I also liked the D7000 VF but once you get used to the gigantic one in D700, it's hard to like DX viewfinders any more.
CF memory cards. I could easily take my SD and put it to the laptop's slot, but in the D700 I had to usb-connect it to the PC. Not a big difficulty though.
JPEGs. You could set the ADL to High or Extreme on the D7000 and get very nice jpegs with very wide DR in the D7000. You could get a nice blue sky in a sunny day in harsh light. That was just a magic of the advanced technology and the very good Exmor sensor. It came in handy in situations when I didn't want to PP or didn't have time to. The same thing you cannot get in the D700. You get noisy jpegs.
I am now also condiering to regather my equipment. I am longing for my D700, but the latest DX cameras, namely the K5lls and the D7100 are tempting too. The D700 was just too large and heavy with good FX lenses, and you can't slap a 85mm on it and go candid in street shooting, mainly because of the wider FOV and louder shutter. But once you taste FX it is hard to go back and I think I will resume my photography with D700 anyway.
