While my TAv range is typically set at 3200 maximum, I view the issue as extremely variable. Generally, in very dim light shooting a scene that is static, you want to keep your ISO lower if possible and lower shutter speed accordingly. Once you go past 1600, the hard-wired noise reduction starts to really show up and you lose considerable detail. In good light where you need a higher shutter speed (typically shooting sports), you'll be amazed at how good the quality remains even at 6400. I wouldn't venture past that ISO, though.
Oddly enough, I found out about this while shooting with the Q - and by mistake. I had left the ISO at 1250 by mistake and took some outdoors shots in the late afternoon. As you can imagine that ISO typically is pretty darn marginal for that sensor, but in good light you can still get a pretty decent result - as you can see here:
Last edited by ScooterMaxi Jim; 11-22-2014 at 04:34 PM.