If you're used to shooting with an everyday DSLR, the Nikon D4 is like holding fully-automatic machine gun for the first time.
Pulling the trigger on the D4 is wonderfully satisfying. When you're shooting in continuous mode, frames peel off super-fast thanks to Nikon's new EXPEED 3 processor, which gets you up to 11 fps in RAW. The camera has a big buffer for continuous shooting, but I learned that the new processor is so powerful that it will capture up to 50 RAW frames before even going to buffer. I didn't push that last claim to its conclusion at the Nikon CES booth, but I held down the shutter release for a while without the camera choking up.
Compared to inexpensive DSLRs the Nikon D4 is big and heavy, but it's actually much more manageable than other professional cameras. It's light enough to use with one hand for maybe a minute, but make no mistake: This is a lot of camera. Don't plan on hanging the D4 from your neck all day. It fills up both of my hands when I hold it on the bottom and from the grip on the right side. To really get the most out of the camera's programable dials and buttons, you're going to want to hold it this way anyway.
Other notes: The camera sports an impressive autofocus that adjusted instantaneously on the show floor. I watched some of the HD video shot with the camera blown up to the size of a wall and it is gorgeous. Based on what I could see on the LCD, the camera makes good decisions and takes great photos in automatic and priority exposure modes. On the brightly lit show floor I couldn't try the low-light performance we've heard so much about, but I'm looking forward to seeing it in action when production models are available.
We obviously can't say anything definitive until a full review, but, it's pretty plain to see that the D4 is going to be a powerful workhorse of a camera. Unfortunately, if you don't have $6000 lying around you're never going to own one.