Apart from the lens performance, the high ISO shots are simply great - even at 12.800, they are better than the 1600 on my 7D - I change everything for me based on how. It takes time to get used to the controls and features, but I love the customizations possible and have already adjusted with the controls I think I need. There tons of videos out there about how to set it - take some time to watch and it will save you tons of time. Also, don't forget to set a control button to turn on APS-C crops, which instantly provides more than 50% reach at the expense of a reduction of about 30% in resolution, but it's post-processing. Hit the trimming.
After spending eight days in Sedona and Grand Canyon, I'm even more impressed. I published almost every shot for highlights, believing I could bring out the shadows without noise, and I wasn't disappointed. I have attached an example where the shadow looks black in the viewfinder.