I have an S95 and seriously thought about upgrading to the S100 because of the new sensor. At this point I'm glad I did not upgrade . From DPreview:
Canon's new purpose-built CMOS sensor in the S100 is capable of capturing images that are detailed, nicely saturated and very clean, especially at the lower end of the camera's ISO scale. The new CMOS sensor gives slightly better image quality than the previous-generation 10MP CCD - an improvement that is subtle but noticeable, especially at higher ISO settings. While the 2MP increase in sensor resolution over the S95 is modest in terms of the additional detail that the camera can capture, the 20% increase in total pixel count does help to offset the effect of noise at a given display size/magnification.
Although the S100 has a broader (24mm - 120mm) zoom range than its predecessor the S95, we've found that the new lens is not as uniformly sharp across the frame. We've tried no fewer than five sample cameras for our studio testing, and in the worst cases significant decentering of the lens causes one side of the frame to be noticeably out of focus in our studio scene (shot at a subject distance of approximately 1 meter). In all cases, moderate to strong softening occurs at the edges of the frame. This is very apparent in our studio comparison tool, where it is easy to spot any visual discrepancies at a pixel level. However, this does not tell the whole story, in our real-world sample shots decentering has been much less of an issue and most of our shots are not overtly blurred by the lens (take a look at the 'lens' page of this review for more detail).
A cause of genuine frustration though is that even though Canon has extended the range of ISO settings on the S100 up to 6400, when auto ISO is used, the ISO sensitivity span is capped at a maximum of ISO 1600. It is possible to adjust the auto maximum from ISO 400 to 1600, but it stops there. If you want to shoot in those higher ISO settings you'll need to select it manually in any of the PASM modes.