I posted on another thread, but will ask the question here as well: if the autofocus is the only major upgrade, is that enough for people to buy the 5DIII?
If the price was the same as the 'usual' 5DII price (before the recent drops, i.e. ~US$2500) and the AF was substantially improved, I would strongly consider it instead of the 1D X, especially with the second joystick on the battery grip. 12 fps would be nice, of course, but I don't have a real issue with the current 3.9 fps.
For me, it will depend on the nuances. I'm fortunate that budget isn't an overriding consideration - yes, I'd like to spend only $3K on camera+grip and leave $6K for lenses (differential from 1D X plus proceeds from sale of 5DII). But it would depend on how much of an upgrade the AF got, and probably also how improved the 1D X's ISO performance really is - Canon has claimed 2 stops for JPGs over the 1DIV, but then they've also claimed, "A new photodiode structure with an increased photoelectric conversion rate increases the sensor's sensitivity by approximately 2 stops over previous models, meaning higher ISOs with the lowest noise of any EOS digital camera." So, which is it? If it's two real stops of improvements due to sensor enhancements and thus reflected in the RAW files...an ISO 12800 that delivers similar noise as ISO 3200 on the 5DII would incline me to the 1D X.