And there you have it. The first tests of the GH3's ISO performance have hit the interpipes. They are good. I won't say spectacular because, first, they are only JPEGs and not RAW, and two, because even with that considered, they are only what we would expect.
Based on my "seat of the pants" analysis, the camera is a stop better than the GH2 at high-ISO. It also lags the E-M5 slightly. This is an admittedly limited analysis, but from what I'm seeing, I don't think there will be much to criticize. That doesn't mean there won't be anything to criticize.
At the minimum ISO of 200, there is some color banding in the shadow areas. And considering that this camera will be going up against the lower-priced Fuji XE-1, which is two full stops better, this is a point of serious consideration for anyone who's planning on buying into a system. I'd say that the ISO performance is good enough where Sony's APS-C cameras hold no specific advantage and if not for Sony's dedication to innovation, I'd say that m4/3 was the obvious choice.
All that said, the GH3 is a unique camera. No other company is even close to offering what this camera has. Based on these photos, I'd say that if you are only interested in still photography, then Sony or Fuji is the better bet. They simply have a fundamental advantage in the form of their sensors. But if you like shooting both stills and video, the GH3 is looking more and more like the camera for you.