I just upgraded from a Canon 20d I purchased in 2005 so this was one heck of an upgrade!
I was astonished with the high ISO performance. Its not exactly a night vision device, but its still amazing to me! I don't have an external flash, and I intend to get one someday, but high ISO performance is really making this a low priority for me. I'm also much less worried about not having fast f2.8 lenses in my inventory.
Going from 5 fps on my 20d to 4.5 fps on the 6d isn't really noticeable, though admittedly I have been focusing on landscape photography lately. I would like to photograph the occasional airshow or eagles in flight and I don't think the half fps drop will kill me.
I do prefer the "joystick" or "nub" on the 20d over the interface on the 6d, but its not a show stopper.
I think the GPS and wifi are both nice-to-have features, though they aren't game changers either. I would like to do a little more astrophotography where I hook my camera up to my 10" telescope. The wifi will allow me to avoid dangling the line from my remote shutter release which can help eliminate unwanted camera/telescope shake.
I find the center focus point low light focusing ability of this camera amazing! This is a huge improvement over the 20d. While this camera isn't known as an action camera in terms of focusing ability I do believe it can satisfy my limited needs in this area. Again, I have to admit to limited testing in this area so far. I've also never experienced high-end focusing capability so its possible I just don't know what I'm missing.
So far I suspect that any problems I've had are a result of a lack of skill rather than a short coming of the camera. If I could pick one or two features from the higher end cameras to add to the 6d, I think my first picks would be 1/8000 shutter speed and maybe high end action focusing.
I paid $2399 and got the bundled 24-105 lens as well as a couple other extras.
I fully expect to use this camera for the next 5-8 years. I can't imagine what kind of performance I'll be getting when I make that upgrade.