Thanks to the OP -- forums like these so often devolve into competitive fanboy sour grapes on hardware decisions, so it's a pleasure to share what is actually great about our gear.
As an enthusiast (read: not a pro photog!), my experience upgrading from crop to FF with the 5D3 was as good as billed. So please take my comments with a grain of salt as I am largely in love with my camera.
So, what do I love about my 5D3? Let me count the ways (in no particular order):
[list type=decimal]
[*]The AF system is a joy to use. Having the 1Dx's AF system is, quite simply, a gift. This single-handedly justifies the price increase of the 5D3 over the 5D2 and 6D.
[*]The large 100% FF viewfinder is such a treat to use that going back to my crop feels like going back in time, like "So this is how people shot so long ago -- how quaint." I love all the grid options as well -- very helpful for 'horizon parallelism challenged' shooters like myself.
[*]The grip is the best I've ever used, hands down. A big piece of this comes from the thickened hand grip compared to my old crop, but in addition, both the grip shape and materials are spot on. The thumb rest/grip feature is so thoughtfully designed.
[*]Low light performance has been as good as billed. I routinely shoot ISO 6400 with minimal cleanup needed, and I have no hesitation to climb up to 10000 should the situation demand it.
[*]It often goes overlooked, but moving from crop to a 5D3 is like moving from kit glass to an L series lens. The build quality is stellar, and so many things we don't think about -- weather-sealing, magnesium body, etc. -- will help this camera survive whatever I put it through.
[*]The silent shutter mode is fantastic. I know it was meant for weddings/events, but it's great for shooting candids, kids, etc. Also, when I finally grow a pair and commit myself to street shooting, that silent shutter might minimize any ruffled feathers of the subjects in frame.
[*]I love the SD card option. Card readers suck, and so many friends and family members have laptops with SD slots built-in. Sharing JPG shots is a snap. I don't shoot video, and rarely cover sports, but it's good to know that I can slide to higher speed CF if ever want to.
[*]The in-viewfinder level is a great feature. See my grid comments above.
[/list]
More will hit me later, I'm sure.
In all, it's a stellar camera. I really cannot find fault with the camera other than some super picky little make-it-perfect-for-me things that are, candidly, too much to expect Canon to give me.
