There are plenty of other threads speculating about whether to swap their XXD/XXXD bodies for a 1D of some kind or another, so having just added a 1Dmkii to the bag, I thought I'd offer up some initial thoughts.
Firstly, speaking as an old MESuper user, it is great to finally have an old school exposure meter up the side of the viewfinder, and having the same for flash exposure is superb. When a newcomer to photography is struggling to master M mode, or manual flash control, it's all much clearer when all the info is laid out for you like it is on the 1D. Controlling the flash especially becomes much more intuitive.
For those that haven't used one, the selecting of menu options on the 1D is also very different. Across the menus, you need to hold down a 'select' button whilst scrolling to the item you want, then release the initial button. This means that it is virtually impossible to alter/delete something accidentally (so far).
Getting used to the difference in crop factor is not too big an issue but it's definitely noticeable. For me, it's made the 24-70mm far more useful at the wide end.
45 focus points... certainly a bit of a step up, and being able to register a single focus point is handy, so if you were using an off-centre focus point for one shot, a single button press can ping you back to the centre AF point or another of your choice.
And then there's the focusing. It's fast. I may be slightly biased towards my lovely new best friend but I was shooting sports and hadn't noticed I was still in one shot mode!
Anyway... yes it's heavier but it makes it so much easier to take control of every aspect of your shooting and that is exactly what I was after. Hope this may help anyone in the same boat as I was.
