As with the NEX-7, most of my issues with Olympus' images are with the JPEG processing rather than the noise profile. You can see the noise reduction kick in aggressively between ISO 200 and ISO 400 (compare the text under the bill).
The E-M5 handles bright, saturated colors very well in both raw and JPEG; even just the red channel of this image doesn't look blown out, thereby preserving detail in highlight areas, as I see with a lot of cameras. Olympus also defaults to a neutral color profile and generally delivers saturated but still quite accurate colors.
(1/200 sec, f11, ISO 200, AWB, spot metering, Panasonic 12-35mm f2.8 lens at 35mm)
Here's an example of Olympus' aggressive default sharpening. It's also so broadly applied that it tries to force areas that perhaps aren't in focus into relief. This is both a blessing and a curse; photos that might otherwise have been a little too soft become usably sharp, but in-focus shots look oversharpened.
(1/250 sec, f6.3, ISO 200, AWB, spot metering, Panasonic 12-35mm f2.8 lens at 35mm)
The camera's sensor does a pretty good job preserving detail in blown-out areas. Shadowed areas, however, can't be easily recovered without introducing a substantial amount of color noise, regardless of ISO sensitivity.
(1/100 sec, f6.3, ISO 200, AWB, spot metering, Panasonic 12-35mm f2.8 lens at 35mm)
With JPEGs, I wouldn't shoot above ISO 800; ISO 200 if you're really picky about detail. But you can probably go as high as ISO 1600 for decently lit shots -- there's too much clipping in the shadows on dark images to recommend this camera for midrange ISO sensitivities, though.
(1/60 sec, f6, ISO 1600, AWB, spot metering, Olympus 12-50mm f3.5-5.6 lens at 43mm)