A great full-frame sensor and excellent lens result in stunning photo quality, even in low light.
Noise and JPEG processing
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.
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.