The X100 delivers extremely clean images as high as ISO 800, with usable ones as high as ISO 1600. I was most surprised by the quality of the camera's JPEG processing; I couldn't easily get better noise-reduction results from processing the raw version, which is unusual.
My only complaint about the X100 is that the sensor doesn't handle reds as well as I'd like; I noticed hue shifts in very saturated reds, as well as the tendency to blow out details. This shows a 100 percent crop of the red channel of the inset image, with almost all detail obliterated.
(f5.6, 1/125 sec, spot metering, AWB)
One of the most difficult things for digital cameras to resolve is massive areas of detail like grass and leaves in the distance. Though not perfect, the X100 does a great job of it.
(f11, 1/35 sec, ISO 200, evaluative metering, AWB)