The SD890 IS displays Canon's trademark noise-suppression profile: there's practically no color noise, just color smearing at the highest ISOs where the blurring gets intense. For very detailed shots, I wouldn't shoot above ISO 200 because of the camera's slightly soft images. You can start to see detail degradation at ISO 400.
The SD890 IS' lens exhibits some noticeable, slightly asymmetrical barrel distortion at its widest; since it's widest is a relatively narrow 37mm equivalent, that's a bit more distortion than I like to see. Still, it's about average for this class of camera.
For compact cameras, I expect to see some purple fringing on the very high contrast edges or those near the sides of the frame, like that of the upper right inset. But there's fringing on most of the lighter edges in the SD890's photos, even in the middle, as shown in the lower left inset.
For relatively low detail shots such as this one--taken at ISO 800--where you're less likely to notice the slight smeariness, you should be OK shooting at higher sensitivities because of the SD890 IS' good color consistency.