ISO 100 offers sharp edges, good detail in shadow areas and is free of noise.
ISO 200 shows no visible difference from ISO 100
ISO 400 is still free of noise with very good shadow detail.
Whereas many compacts start to become unstuck at ISO 800, the X10 is still producing clear shots.
At ISO 1600 shadow detail begins to soften as the X10's noise-reduction processing kicks in.
At ISO 3200 the image has become noticeably softer, but the X10 can still produce usable images which is hugely impressive for a camera of this size.
ISO 6400 is where things start to really break down.
ISO 12800 isn't somewhere you want to go if you can possibly help it.
A real-life example of a perfectly usable shot taken at ISO 1600. Click to expand to actual size.
And the same image once more, this time with the X10 set to ISO 3200. Again, click to view at actual size.