It's not just for very low light...if you need speed beyond what the light will give and also want to stop down for better resolution, the K5 will let you do it with ease.
ISO 6400 F10 1/160
Or in very dim daylight/cloudy conditions you can shoot wide open and still get some speed.
1/125 F6.7 (wide open Bigma) ISO 12,800
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Either way you can get a shot you might have missed otherwise, or a better shot than you might have gotten at a slower speed. They aren't all success stories, but most of them are much better than no shot at all. It does change the game considerably.
Regards!