The combination of the full-frame sensor, great lens, and solid JPEG processing delivers extremely clean photos up through ISO 800, very good at ISO 1600, and reasonably good at ISO 3200. I wouldn't recommend shooting ISO 6400 or above in JPEG.
ISO 6400 is usable under certain circumstances, but I don't recommend ISO 12800 and up for JPEG unless you'll be displaying the photos at reduced sizes or are willing to accept the tradeoffs.
While I could get lots more detail in the images by processing the raw at ISO 3200, that's partly due to the much higher contrast of the JPEGs thanks to the default settings.
JPEG versions of ISO 3200 images produce nice dark skies without artifacts (bottom), mostly because the standard settings push the contrast so that the shadows clip. In the raw, more tonally accurate version, you can see the noise in the sky.
The RX1 produces extremely pleasing colors, but for accuracy you need to change the settings. The automatic white balance is better than on most cameras, however.
As with most of Sony's cameras, the default Standard Creative Style pushes the saturation and contrast until the hues shift. Neutral does a more accurate job, without losing any sharpness.
I accidentally switched the camera into the Vivid Creative Style while testing and thought I might as well provide an example; here's what the colors and contrast looks like.
