It handles so much differently than any dSLR camera I've previously had (20D, 5D2, and 5D3). You can tell it's weather sealed by all the small details that are needed to let's say open the CF card compartment, or insert the battery, etc. Needless to say this thing is a monster... big and heavy. I'm sure people who manhandle 800mm lenses will not have a problem with this. Hey, at least it's good exercise, right?
The rubber feels better than the 5D2 and the 5D3. It fits the contours of my hands just as well as the smaller, grip-less 5D-series cameras, despite its larger size and heavier weight. I personally saw a noticeable difference in build quality between the 5D2 and the 5D3, where the latter was better. That being said, the 1D X makes the 5D3 feel like a toy... literally. It's solid all the way through. The buttons feel better. The toggles feel stronger. And not to belabor this point, but there is a marked improvement in weather sealing. It looks and feels like a tank that can easily take some hits.
Now keep in mind, this is my first pro 1D-series camera, so I can't say right now how it compares to the 1D IV since I haven't played with it. One of these days I'll go down to my local pro camera shop and mess around with it a little to compare.
Anyways, since I took some Tylenol PM a while back, my brain is shutting off. So, I guess I'll turn it on and play with it sometime tomorrow. Hopefully I get over these flu symptoms quickly, because I really want to go out and shoot!