The SX10 IS' photo stay relatively sharp and noise free through ISO 200. At ISO 400, a bit of blurring kicks in and by ISO 800 you lose most small details and start to see some mottling. Overall, however, the SX10's photos do visibly improve over the S5 IS.
It's kind of interesting to look at the difference in noise reduction between the SX10 IS (left) and S5 IS. This is a 100 percent crop from the blue channel of the ISO 1,600 images from both (I boosted the gamma for better visibility). You can see the hard-edged, blobby artifacts of Digic III gives way to the streaky lines of Digic 4.
In an improvement over previous lenses, the SX10 IS shows some fringing from the lens distortion (left and right insets), but not nearly as bad as it's been in the past. Because of its relatively low resolution, there is some muddying up of background details, like the net over the door's facade (middle inset). (ISO 80, f3.2, 1/80 sec, 28mm equivalent)
As with many cameras, the SX10 IS seems sharpest in its Super Macro mode (macro fixed at 28mm equivalent). Here you can see that the sharpness also holds up pretty well at ISO 200. (f2.8, 1/50 sec, ISO 200)
Canon hasn't entirely fixed the tendency for the megazoom images to look alternately crunchily oversharp and diffuse, overall they're pretty good. (ISO 80, 1/100 sec, f3.2, 28mm equivalent in Super Macro mode)