When the light is low, the ISO on your camera has to be raised, and the result is varying amounts of noise in the image.
The Fuji X100 copes with high ISOs better than any other camera I own. Not surprisingly, better than the GF1, but also better than the Canon 1D which tends to lose colour definition at higher ISO ratings.
The light at Doi Suthep temple this weekend was appalling. The whole area was covered in mist and clouds, and high ISOs were required.
This shot was at ISO 3200. Not only was most of the noise removable with only a small adjustment in Lightroom, but the camera also did a fine job of retaining all the colours.
Very impressed Fuji, now sort out the abysmal manual focusing please.