Indeed, once you go for a 1-Series you don't go back...
I have a 1D II and a 1Ds (looking to upgrade to a 1Ds II). The price second hand 1D Mark IIs are going for are simply incredible, I've seen them locally for $2000 - these are insane cameras for the price.
The build quality is second to none, the weather sealing means you can hose your camera off if its been in mud/dirt and the ergonomics are fine once you get used to them. There are so many little features of the 1-Series bodies which make my opening statement true for me at least - the file name LCD is fantastic when your shooting social events and you need to take file names for captions, the voice recording is great when you have to caption, you can quickly "reset" the camera settings to a preset
mode/ISO - great when you pull the camera out the bag ensuring your not shooting at ISO3200. There are also a plethora of focus options including an "extra" button near the "*" which I've tied to the centre focus point (couldn't live without this).
I went from the 20D to the 1D - for an all round camera, its simply incredible. Everything the xxD does, the 1D2 will do much better. Having actually not used a 1D Mark II N for more than a fraction of a second, I really can't comment on the real world differences but at the end of the day, a 1D 2 is a damm capable camera especially for the price.