There doesn't seem to be any completely noise-free ISO sensitivity--even at ISO 100 you can see stippling in some shadows--and while there's little sharpness drop-off by ISO 400 a bit of contouring begins to appear in dark areas and the color noise becomes more pronounced. Still, for its class, photos remain generally acceptable through ISO 800 and usable through ISO 3,200 depending upon scene content.
At ISO 800, shadow areas appear a bit stippled, but the overall photos still look pretty good. Beyond that there's a visible decrease in dynamic range and increase in color mottling, but the photo quality fares quite well for the price. (1/320 sec, f5.6, ISO 800, 14-42mm lens)