It's a game-changer my travel work! I "won" an EM-D5 II last year and was thrilled to have it!
What others say about having to use a tripod and static subjects is very true. Moving water just doesn't look right and the movement is a little weird... That said, I shoot in high-res raw files always with a tripod. I shoot ancient Orthodox monasteries and churches in my travels, specially iconography and specialty round-fisheye photos. The EM-D5 II has now replaced my much larger system, especially for travel.
From what I understand from the way it does it's thing in the camera, 8 images are captured in a short time frame and then processed into one file, perhaps without the color interpretation required of the Bayer filter mosaic technology.
In shooting my fisheye, a Sigma 4.5mm 2.8, IMO, the 64mp raw files are nearly comparable to shooting my 8mm 3.5 Sigma on a D800. I've created other photos using both the Olympus and Panasonic lenses and the files are still nice. Granted, the files are different between the two systems but for the size, very comparable.
The high-res mode limits ISO and aperture and I think the latter has to do with programming to limit diffraction limitations of the lenses. For my fisheye and adapter, totally manual focus and aperture, the aperture isn't limited as most likely with any adapted lens.
Some of my photos require HDR so the files start to add up. Raw files come in about 100MB plus the paired JPEG file.
The main advantage for me is that my travel kit is now about 1/2 to 2/3rds the size and weight of my FF Nikon system and gives me more flexibility working in the field.
If you are careful and use good craft, the high-res setting on the Oly is phenomenal, especially for such a small camera!