Our lab test photos actually seem to represent a best case of the SX20 IS' image quality, although our noise numbers do reflect my field testing results. Based on our test shots, I'd say the camera fares well at ISO 80 and ISO 100, and begins to display a loss of detail starting at ISO 200; by ISO 400, detail visibly degrades.
This was typical of my outdoor photos with the SX20 IS (though having this squirrel run up my leg in the process was pretty atypical). At small sizes, most photos look sharp, but when examined closely--or printed at 12x16--they look noisy and mushy. This was shot as ISO 200, but the ISO 80 photos look just as bad, and our noise numbers confirm that the camera's noise at ISO 80 and ISO 100 are unusually high, even for a class of camera known for image problems. (1/80, f5.0, ISO 200, AWB, evaluative metering, 289.5mm equivalent)
Ironically, while noise at lower ISO sensitivities is higher than the SX10 IS', the SX20 IS actually fares comparatively well at ISO 1,600. That's not to say that the photos are great--they're obviously soft, with the yellow blotches indicative of blocky noise reduction in the blue channel--but the color remains relatively saturated and the photos may be usable at smaller sizes. (1/60 sec, f2.8, spot meter, super macro at 28mm equivalent, AWB, ISO 1,600)
Canon's version of its automatic exposure correction feature, i-Contrast, works pretty well, and doesn't seem to introduce any visible noise or artifacts. (1/60 sec, f5.0, ISO 80, 295.7mm equivalent, AWB, evaluative metering)
Since it uses the same lens as the SX10 IS, I was surprised to see more fringing in the SX20 IS' photos; I think its the lens/sensor combination. In any case, here you can see both the purple and green halos, which aren't restricted to high-contrast edges.