If you really need to use ISO 1600 or above, I strongly suggest you shoot raw.

Any camera can have noise at low ISO sensitivities which will come out in low light conditions or if a shot's underexposed. What's important is how much flexibility you've got to improve the exposure without bringing out the noise. This isn't an egregiously dark shot, but you can see how bringing up the exposure starts to show a little color noise. I could probably have tweaked it a bit more with some time.

(1/80 sec, f2.0, ISO 80, evaluative metering, AWB, 4 inches away, 24mm; inset is the retouched shot.)

For a low-light scene like this, the PowerShot 100's JPEG noise-reduction algorithms do a pretty good job. You can see a bit of color noise on the whites of his eyes and some fuzziness on edges, but otherwise the photo is quite serviceable.

(1/25 sec, f2.0, ISO 800, evaluative metering, AWB, 24mm)

Canon does seem to have improved its noise-reduction algorithms from the S95, although it's also possible the S100 just has a cleaner sensor. Whatever the reason, the S100's ISO 1600 shots look better than the S95's. That said, the S100 also seems to expose a little darker and slightly warmer than the S95.

(1/25 sec, f2.0, ISO 1600, evaluative metering, AWB, 24mm)

Canon's noise-reduction algorithms remain some of the best I've seen--that is, you can do a little better at avoiding edge artifacts and sharpness loss by processing the raw files yourself, but only by making some tradeoffs with luminance grain.

(1/25 sec, f2.0, ISO 1600, evaluative metering, AWB, 24mm; raw processed using beta codec in Adobe Camera Raw.)

As with many cameras, noise-reduction and compression artifacts tend to become exacerbated around the edges of the frame where lens distortion makes it difficult for them to work properly. This isn't unique to the S100, but because of the longer zoom range and wider angle of view, the lens has more distortion than the S95 and you're bound to notice it a little more.

(1/25 sec, f2.0, ISO 1600, evaluative metering, AWB, 24mm)

Canon's multishot HDR mode isn't very good for expanding the tonal range of a high-contrast scene: you really can't use it handheld and you simply don't know when it's safe to move the camera again because of the mystery final longer exposure length. On the other hand, it's kind of nifty to use for shooting traditional multiple exposures.

The lens delivers pretty sharp images, even at a higher ISO sensitivity, and without looking crunchy from oversharpening.

(1/50 sec, f4.0, ISO 400, spot metering, AWB, approx 49mm equivalent)

Canon (like all camera manufactuers) obviously does in-camera defringing, but it clearly shows up in the raw files and it's not that easy to remove.

(1/30 sec, f2.5, ISO 80, evaluative metering, AWB, 24mm equivalent)

While you can see some left-biased barrel distortion, this isn't bad for a 24mm lens. And as far as I can tell, it's not being corrected in-camera, since the raw files look the same (nor is there a menu option for it).