I've been shooting with 1ds3's since they came out and have also wondering what next.
Will also admit that I hadn't seriously considered the 1Dx because of it's slight drop in resolution while everything from Nikon
seems to be doubling..
Anyway, for the last few days I've had a loan 1Dx and would agree it's a big step forward over it's ageing parent.
The high ISO performance is awesome and while I would limit low light shooting to around 1000 ISO on the 1DS3, the X is still
kicking it well above 3200..