Overall photo quality produced by the S640 is very good. Many cameras in its class suffer a significant dip in quality when they use any sensitivity above ISO 200. The S640 is actually good to ISO 400, though there is some visible noise and increased softness. The camera lets you limit the auto ISO range to either 100-400 or 100-800. If you're in daylight or bright conditions, I recommend locking it down to 100-400. It performs OK up to ISO 800, but there's definitely increased softness and noise/artifacts. The S640 can shoot at full resolution up to and including a sensitivity of ISO 6,400. However, everything from ISO 1,600 up doesn't look good because of color shifting and yellow blotching and the near absence of detail. So while you can, in fact, keep shooting in low-light conditions, you probably won't be thrilled by the results.
Taken at ISO 100, you can see just how good this camera can be. Great color, detail, and sharpness. The results at ISO 200 are similarly good. But again, go up to ISO 400 and you'll start to lose detail and get faint yellow noise.
Nikon does a very good job of controlling lens distortion. There is almost no discernible barrel distortion at the wide end of the lens (top) and no pincushion distortion at the long end (bottom). Sharpness is fine in the center, but softens a bit in the corners. Also, purple fringing is within acceptable amounts for its class.
Colors are not entirely accurate from the S640 with reds and blues a little pumped up, but the results are pleasing. Everything turned out nice and bright and reasonably natural-looking. Typical of compact cameras, highlights tend to blow out, but at least Nikon's D-Lighting system helps bring up shadow detail.