I'm a recent 5D mkiii cult member and I really like it. I'm not quite sure that I buy into all the hype that full frame takes your photos to the next level and you'll look back at your old crop sensor photos with disgust and loathing. But I do like the mkiii. I feel it is a more than adequate upgrade from my 60D and it was well worth the money it cost to upgrade (Cost of mkiii minus the money I received from the sale of my 60D).
I haven't fully tested the low light performance of the mkiii though I will acknowledge it is superior to the 60D and the frames per second is more than satisfactory, and the auto focus is excellent... Basically I don't have any REAL complaints about the mkiii where as I did have some issues with the 60D (not afma, low light performance, rubber grips starting to come off, etc.) and I didn't want to upgrade to the 5D mkii because of the AF performance and the lack of the cross type peripheral AF points and the meh shots per second rate.
But my contention is that the 5D mkiii is practically the perfect mix of performance and durability for me. So why wouldn't I keep it for 7 years (when then mkV is released and presumably the mkiii is about to die)? What features would drive me to consider upgrading prematurely and by extension, what features would drive yall to upgrade?
The launch price for the mkiii body was $3500 which sounded astronomical at the time. If the mkiv's launch price is $4000, but you sell the mkiii for $1700 making the upgrade "only" $2300.
So the aforementioned question is 'How much better would the mkiv have to be for you to upgrade from the mkiii'? And I'm sorry that this basically excludes all non-mkiii owners, but ya'll haven't drank the Kool-aid yet.