The 6D is an upgrade from my beloved 60D. Resisted letting go the 60D for years as I am a casual user and tried to focus on glass. Also my grab-and-go lens was an EFS - making the switch a kind of expensive. However, having and using the 6D for a few weeks and 1000 pictures later I realize that I should have switched long time ago. I am blown away by the improvement of IQ and especially for low light performance.
My main use: Mostly event and party photography, non professional - just the only guy at the party with a DSRL. No need for fancy and fast focusing and fps is irrelevant for my purposes. The 6D is ideal for this.
Ergonomics: It's almost the same body as the 60D - no need for me to relearn anything. However, there are two little nice improvements: The physical wheel lock to avoid using the front wheel unintentionally and the physical lock for the film/movie modus. Both VERY useful.
Picture IQ: This is the thing that I am absolutely stoked about. Whereas on my 60D the picture quality deteriorated fast after 800 ISO - the 6D has another 3 to 4 stops to go. Many times I carried my flash along - just to realize that with the new body I can handle the situation without flash just perfectly.
The only gripe I have with the camera is the missing on-board option to remotely trigger speedlites. The missing on-camera flash and no radio control option requires an additional gadget to take along for controlling external flash.