It buys you more sophisticated auto-focus, including the ability to auto-focus on a lens with a widest aperture of f8 (5D is limited to f5.6) - such as an f4 lens + 2x teleconverter.
It buys you a built-in portrait grip (extra $ on a 5D).
It buys you part of the way to CPS membership (need two 1-series bodies).
It buys you the ability to write photos to both a CF card and an SD card simultaneously for backup.
It buys you a really sore neck (the 1Ds Mk II weighs about double the 5D)
However, I wouldn't buy a 1Ds Mark II (unless second-hand) - I'd hang on another half year and get a 1Ds Mark III instead - judging by the 1D Mark III it should have a stack of awesome improvements, including rather more than 4Mpixel over the 5D.
I'm using a 5D right now, but I intend to get a 1Ds Mark III as soon as possible.