Focus Numerique has posted a JPEG ISO comparison on its website including the new Canon G1X.
First, the camera performs fantastically. It is, as would be expected, bested by the Sony NEX-5n. The interesting bit, though, is its sensorial and nominal brethren, the GX1. All things being equal, they should be nearly identical, and they aren't.
At all ISO's, The GX1 is outperformed by at least half-a-stop. By ISO 1600, the performance difference has extended to 3/4 stop. That is embarrassing for Panasonic. The Canon manages to produce Facebook-usable photos at ISO 12,800. If the dynamic range and color depth of the sensor holds up, this sensor will stand among the current crop of APS-C sensors on the market. The Panasonic looks terrible in comparison.
Now that I have sufficiently ragged on Panasonic, the bad news. The sensor is great, but the camera will likely not be worth $800. Even at f/5.6, the aperture at which all of the pictures were taken, the lens shows significant softening in the corners, devolving into some extreme softening at the edges.
Compare this to the lens on the Fuji X100, which FN has on their comparison page, or the 25mm lens on the GX1, and the difference is inexcusable. For the price, I was expecting significantly better lens performance. This is quite a disappointment.
UPDATE: I realize that the lenses on both the Panasonic and the Fuji are primes and the G1X is a zoom, but when Olympus has upped the bar with the XZ-1, this lens design no longer flies.