Utterly superb. Although it hasn't been out of the bag in a week, but going from a 5dMK3 to this is a huge jump. The amount of details per shot is another level, and I can really see a lot more detail in my landscape stuff, especially the smaller stuff in the distance. There also seems to be a lot more leeway with shadows and highlights.
ISO wise I've not really gone above 100 yet so can't comment.
For those worried about memory cards, I've got 2 SDXC cards for it, I have a high speed UHS 300MB/s sandisk extreme pro (£100 for 64GB) and a Sandisk Extreme 128GB (£28!!) 150MB/s card. The slower speed is perfectly fine, I've done a few test burst shots with it and it handled everything fine, yes there was a delay waiting for the buffer to finish writing, but I held the shutter down for a good 10 seconds or so with no apparent slow down.
Oh and the EVF is a thing of beauty!
No comment on video as it's not something I shoot (not even tried it yet)