I just upgraded from a 550D to the 6D..... World of difference when it comes to ISO performance, speed, overall quality.
I am able to focus and take reasonably usable shots in a room dark enough that you can barely see through the viewfinder - my 550D would have just sat there and racked the lens back and forth. In the ISO department, my personal limit on the 550D was 1600, and I really didn't like going past 800 - with the 6D, it seems like I can go 6400 and still have better IQ than the 550D, and in a pinch go higher if I can't use flash (I do a lot of band photography - dark is the status quo). I can see where the 6D could be lacking in sports photography, but in my case it would be more from the lack of the 1.6 magnifier than auto focus behavior - if I could shoot motorcycle races with a 550D (70-200 f4), which has an even more archaic autofocus system, and a slightly slower framerate, then the only disadvantage I have going in, is the lack of reach (so maybe next Christmas a 100-400 will call my name).
It has been a few months since I borrowed a friends 5D mkII, but from what I recall, the shutter seems quieter on the 6D, focusing was faster with 6D, and ISO performance seems to better with the 6D (I compared some 6400 shots from each, there was a lot more noise on the 5DII).
I know that some people say the 6D is a plastic toy - I wonder if they have even held one ? It feels solid enough to me, and with a battery grip and 24-105mm lens, it could be used to bludgeon a mugger.
The 28-135 is a reasonable kit lens (hey they packaged it with the 7D - can't be total crap). I used it a lot on my 550D for kids parties, machinegun shoots (lots of brass comes out the side at a high rate of speed - don't want that smacking my L lenses), and bar shoots. The image quality was reasonable enough for those situations, and the dollars invested in the lens were low enough that damage wouldn't ruin the month. Was it as bright and nice to use as the L glass - no - but it served its purpose.
Am I happy with my 6D - Yes.... and in all honesty, I might just grab a 7D mkII or 70D in the future for motorcycle / boat racing photography, but that would mainly be for the extra reach with whatever lenses I have.