Digital Photography Review has published a full review of Panasonic's Micro Four Third flagship.
Snippets from the conclusion follow:
"In terms of Raw capability the images it produces are sharp and have reasonably low noise levels. There's not a tremendous amount of additional dynamic range to be had out of the files, but they're reasonable malleable. The JPEG noise reduction isn't quite as impressive - with higher ISO images losing all their fine detail."
"Anyone who found the OM-D's handling too fiddly will love the GH3's substantial grip, while its magnesium-alloy build strikes a good balance between offering a feeling of solidity, without adding excessive weight."
"The GH3 has some of the most comprehensive wireless control we've yet seen on a camera of this type, with full control over exposure, focus and the majority of camera settings. Wi-Fi can also be used to automatically download files to your computer or upload them to the web."
"The Panasonic GH3 is the new standard bearer for video on a consumer stills camera. At the time of writing no other camera at a similar price point offers so much for video shooters and filmmakers. Whilst the Nikon D5200 / D7100 and Canon 5D Mark III perform better in low light, neither have the range of video features and frame rates the GH3 offers for $1299 - nor quite the crispness of detail at 1080p."
"For us the GH3's stills capabilities, while good, are not enough to make it stand out - Olympus' more compact E-M5 produces nicer JPEGs, while the likes of Nikon's D7100 offer an edge in stills image quality."