As far as I can tell on my mediocre laptop screen, the E-M5's JPEGs deliver a reasonable dynamic range. I don't yet know how much recoverable detail it retains in the highlights, though, as there seems to be a little more clipping in the highlights in some photos than I'd like.
Note that this image is square because I accidentally managed to switch the aspect ratio settings. That's Shawn Barnett from Imaging Resource knee deep in the snow.
(1/1600 sec, f8, ISO 200, center-weighted metering, AWB, 12mm lens)
The 12mm lens is a nice, sharp lens, and you can see it delivers good midrange detail. But once again, the images look a bit oversharpened to me. That's Josh Root from photo.net pointing the camera right back at me.
(1/1600 sec, f5, ISO 200, spot metering, AWB, 12mm lens)
This isn't bad for ISO 800. It retains a significant amount of detail, and the only softness you see is in conjunction with the falloff in depth-of-field. That said, this is also a close-up shot, which tends to start out sharper.
(1/80sec, f2.2, ISO 800, spot metering, AWB, 45mm lens)