I've got both the Q-S1 and the Q7. The Q-S1 will autofocus while doing video with the 01, 02 and 08 lenses. I think the Q7 is a little more ergonomic, but not by much.
The Q system's greatest strength is in versatility using specialized lenses. If you can get into manual focusing, it will accept virtually any lens ever made, and will make any of them image stabilized. The system is popular with birders and macro enthusiasts because of the crop factor. Here is a
handheld unprocessed jpeg shot with a 90mm APO-Rodagon enlarger lens on the Q-S1:
IMGP0788_zpsgxzipn3q.jpg Photo by dktrfz | Photobucket
As far as low light and action goes, like any small sensor it has its limits, although it also has an advantage: for any given field of view it will have much greater depth of field than a FF or APS-C system. Here's a link to a photo album shot in a night club, the first four pictures are with the 02 zoom and fill flash, most of the others are available light with the Pentax 110 18mm f2.8, there's one shot with the 03 fish-eye as well:
Mystic Funk by dktrfz | Photobucket
I would disagree with 6BQ5 about the Q's jpegs vs. RAW., Pentax cameras in general have very good jpegs, the Q is no exception. You may be able to get somewhat better images with RAW, but I can't, at least not with PS's converter. I find that I get the best results if I don't pre-sharpen the jpegs, turning the fine sharpening setting to its weakest setting on the jpeg preset, then upsizing the image in PS, then smart sharpening as the last step.
I generally limit my ISO to 800, beyond that it gets somewhat impressionistic.