I have been using a D2x since its release. Yesterday, I picked up the D5. I didn't open the box until last night. So, my input will be very limited, until I have a chance to really play. And I can't wait to play.
The first 2 hours was dedicated to charging the battery. The charger that comes with the D5 is a dual charger and will charge the D2x battery (EN-EL4), as well. That is great for me.
Jumping from a D2x to the D5 is a significant jump. While the D5 has much more features, than the D2x, I feel the learning curve will not be as great because, both body and buttons on the D5 and D2x are pretty much the same. On the D5, some buttons were moved to new locations, to allow for new buttons. For example, the ISO button is now near the shutter release. There is 'i' button on the right back, that provides information about the photo. The Mode button is where the BKT button was, the flash button was removed, now the BKT is in its place. There is a 'fn' button next to the vertical shutter release.
I did a test shot, under low light conditions, but I have not uploaded it to my computer to view. It was late and I had to go to sleep. However, I did view it in the camera's display, it looked pretty good.
The D5's menu system is very similar to the D2x's menu system. Again, this is great for me, because it reduces the learning curve. The D2x did not have video capabilities. Therefore, there was no menu section for video, as the D5 has. What was also new for me is the Copyright option. You can add your copyright information into the camera, so that it appears in the files' efix data. I always added it after the fact, via Photoshop. That will save me some post editing time.
The D5 has inputs for ethernet, external microphone, HDMI, USB, headphone. peripherals (ie: wireless lan, wireless remote control). I believe other Nikon cameras subsequent to the D2x had a number of these inputs.
By the way, I decided to go with the XQD version of the D5, because the read/write times are much faster than the CF cards, especially if using the video recording feature. Having 2 memory cards instead of 1, is wonderful. I picked up 2 Lexar 2933x cards (128gb each). I only shoot RAW, so the D5 calculated that I can shoot up to approximately 2.7k photos. I am really gonna have fun with this. But, I really bought the large capacity for video recording.
As I continue to play and use the D5, I will post more information. Based on the forum feedbacks and my own limited experience, I feel I will be really happy with this camera.