I've been a loyal Canon shooter for 15 years now, starting with the AE-1 and Elan film SLR's, then moving through 10D, Rebel XTI, 1D, 1DS, EOS-M, 5D Mark II, and 5D Mark III (my current Canon.) I've invested probably $50k in Canon over these years, and have been able to create beautiful photographs. The lenses are excellent (the L series in particular) and the cameras are durable, reliable and easy to learn the various menus to get things done. The repair depot is right down the street from me so when I drop my camera or lens, I'm able to get it fixed in a week.
The past few years I've seen a huge wave of "Four Thirds" and other mirrorless cameras stirring controversy; the compact size, ability to use other lenses with adapters, and now recently super fast autofocus and high sensitivity/low noise sensors are compelling me to look at making the shift away from my beloved Canon. The slow pace Canon has with technology changes are really starting to impact their line up; the EOS-M which is three years old is their only mirrorless interchangeable lens camera, and it has no electronic viewfinder (not good for me.)
It wasn't until I saw the recent reviews and videos of the Sony A7S that I really got excited. The camera is not a megapixel killer (12 megapixel when others are 24+) however the ability to have "native ISO 490K" with 15 stops of dynamic range changed my thinking. I have been very unhappy with the noise in the shadows of my images, particularly in the studio where I am purposely shooting the extremes of the zones, highlights to shadows. The little red speckles for no apparent reason just irk the hell out of me and require post work to get rid of them. My initial testing of the Sony has blown me away....you could not take a poorly exposed image with this camera. Your shots in difficult lighting will yield detail in shadows and highlights! Tha A7S is a full frame sensor camera, is pricey at $2500, but the flexibility of this tool is just incredible.
I know this is only the tip of the iceberg, and likely as not this technology will eventually make its way into Canon DSLR's if they buy Sony chips...but the ability to use Canon Lenses on Sony e-mounts with adapters has made it possible to use today's Sony without losing my investment in Canon lenses; a great thing when you think about all the other little bits like filters that fit Canon lenses, flash battery packs for Canon strobes (you can use them in Manual mode just fine.) Another great thing: focus peaking! I have waited years for Magic Lantern to get the 5d MK III version of the firmware done (still waiting) so that I can better use the super fast lenses like the Canon 85mm f1.2 L; there is no good focus screen for the MK III so critical focusing is nearly impossible. With the Sony A7S and electronic viewfinder with focus peaking turned on, you can get super fine focus accurately...something I just can't do with the Canon.
I have a mind to get the Sony A6000 as well; the fastest autofocus mirrorless out there, with whopping 24mp sensors....with a Canon 70-200mm lens, you really have an amazing sports camera.