see:
http://www.fredmiranda.com/forum/topic/1400719I upgraded - I shoot birds in flight with a 600mm F4 version II at 800-1600iso
Compared to the 5D3, the colors seem to me more accurate in areas where the 5D3 fell apart - underexposed parts of the subject. I can take a 5Ds file and underexpose and push it 1-2 stops and the image looks great.
I don't miss the 6FPS of the 5D3...5fps is fine with the 5Ds. The biggest problem is the large file size - writing to the card takes longer so if I need to check if I have the exposure correct on the fly, I have to wait a while (3-4 seconds) before the camera processes a file (or a burst of images). Battery life is not as good as with the 5D3 (as expected) so make sure to get third party batteries, etc.
Image quality is improved...detail is wonderful on a large monitor. I am glad I made the purchase - would not go back to shooting BIF with the 5D3...files are indeed better (mostly). Now if they could only get the detail in the shadows/blacks as with a Sony sensor, etc. See third photo below as example of this - the black head of the young Bearded Vulture (Lammergeier) - left side of head in the shade lacks detail in the black area...could be my attempts at processing this file on a laptop on the road...
or as I wrote over at FM: "I have been shooting the 5Ds every day since late October at my research site in the mountains of Nepal. I replaced the 5D3 with the 5Ds...since reviews showed that at least to 1600 ISO, the sensors in the both cameras were more or less equal...with the 5Ds having more random noise and better at lower ISOs (100-400).
"I shoot birds in flight at 800-1600...in processing the RAW files it seems to me (after having used the 5D3 for three years in the same situations) that the RAW 5Ds files can have the shadows bumped (pushed) better than the 5D3...also, I could never get whites to balanced correctly on the underside of a bird passing overhead without a greenish tint in the shadows...or if I made a color correction, with grey areas (secondary feathers especially) becoming reddish (magenta). So overall I would say that for me the sensor in the 5Ds is a noticeable upgrade...not quite Sony sensor quality (I shot the RX-1 for a year), but it is better. And as I always say, I never shoot a Canon camera over 1600 ISO - though I have not yet used a 1Dx (too big, heavy and expensive for the quality)...but will shoot the RX-1 at 3200.
"My biggest complaint is that it takes a long time for the camera to process the files while shooting - so if I need to make exposure adjustments on the fly - I have to wait and possibly lose some shots waiting for the camera...
"Battery life is not as good - I get about one full day (up to 600 images) on a full battery charge.
"AF is IMO more accurate on my copy of the 5Ds than on my 5D3. I don't mind the 5fps of the 5Ds compared to the 6fps of the 5D3.
I think the cost is still several hundred dollars too high - especially given the exchange rate between the Yen and US Dollar. That being said I got mine for $2750 in late August from a dealer in NJ - all has worked out fine.
"Anyway, it was a worthwhile upgrade for me...I think color fidelity is better with the 5Ds...and the ability to crop like crazy (think distant small birds in flight) makes a big difference for me. I would sell the 5D3 again and purchase the 5Ds knowing what I know now...AND keep your RRS L-plate from the 5D3 because it fits the 5Ds 99.9% perfectly."