A couple of days ago I went to a party and I brought the X100s to try it out in a very low light environment. I also had my trusty D800 with the 24/1.4 mounted with me just in case. That was totally unnecessary cause the X100s did just well. There were only a few situations the autofocus refused to lock on and sometimes it needed some time to focus. However overall I am really happy with the results I got. Most of the shots have been taken at ISO 6400 and post was done in LR 5 beta.