Lucy has her little nightly schedules. When it's time for all of us to go to bed she follows me into the bedroom and then hops up next to me for her nightly rubs. Last night I put the GX1 on the night stand next to the bed, so that when she jumped up and settled down I was able to grab a few experimental ISO photos. I took these because of a comment I read about how good the GX1's low light performance was supposed to be, and I wanted to see for myself. I seldom push this camera into high ISO territory, preferring to use the camera at or near base ISO. So I grabbed a number of shots of Lucy at varying ISOs all the way up to the GX1's maximum of 12,800. Light was the bedside light, an equivalent 75watt output Phillips LED.
Top photo was set at ISO 400 by the camera. The other two ISOs were explicitly chosen by me. From a practical perspective ISO 6400 reminds me a lot of old Tri-X at ASA 400. The ISO 12,800 reminds me of when I would push Tri-X four stops to ASA 1,600. In fact it reminds me a lot of old film journalism low-light shots, printed on cheap newsprint. I should note that there was plenty of available light from that bedside light; the shutter speed at ISO 12,800 was 1/2000sec at f/1.8. The top photo's shutter speed was 1/60sec at ISO 400.
Personal experience for me is that I'm comfortable shooting up to ISO 1,600 with the GX1, considering that base ISO is 100. Higher ISOs are "artistic" or "gotta have it, damn the noise" territory. The E-M5 and NEX 5N do at least a stop (ISO 3200) to two stops (ISO 6400) better with regards to high ISO noise. But still and all, if you're running with fast glass (1.8 or faster) and shooting at ISO 1,600 or lower, then all three are pretty much equivalent, and have unique characteristics with their sensors. I should note that the GX1's base ISO is 100, just like the NEX 5N. And at base ISO the GX1's results are quite lovely indeed.
Everything taken with the 20mm at f/1.8. All images straight out of the camera, not resizing, no post processing. If I were doing this for real (define what "for real" means), then I might break down and do some post work on the ISO 12,800 raw file.
After the last shot I put the camera down, rubbed Lucy's head, and then we went to sleep. Sometime in the middle of the night she moved elsewhere, but then, that's what cats do.