LoneRider, definitely give it time to get your technique down with this lens. I have the 6D, and I like you was not thrilled with the initial shots I was getting with the lens. I did end up doing an MFA at the long end and over time, I learned that it was my technique that was the issue not the lens.
Now that I am comfortable with the lens and it's best settings for certain types of shots, I really love the shots I create. I got this shot while my wife was running on the local high school track, while I was sitting on the bleachers. She was running so I had to bump the shutter a bit more than I would normally, but the 6D performance on ISO meant the 400 ISO was not a concern.
This thing is freaking amazing when you get the settings right. I was on AI Servo, continuous shutter, auto ISO at 6.3 (you can see the background starting to bokeh) at 1/800th.

















