As much as I love sharp pictures of motion, for a variation on a theme I will use slow shutter speeds for an artistic interpretation of movement. The question is what shutter speed to use. I typically choose between 1/6th and 1/30th of a second, and that depends on the lens I'm using, how fast the subjects are moving, and how much of the frame is filled with the subjects. The truth is, though, this is a trial and error situation. Sure, based on years of experience, I can make a reasonably accurate guestimate of the shutter that will give me ideal results, but a photographer can never really know the best speed until the initial shots are examined on the LCD monitor. At that point, he or she can tweak the results by using shutter priority and selecting another speed.
For the group of horses above, I used 1/15th of a second, and for the single horse below running past me in sunset lighting, I chose 1/30th. Which speed you use depends on the factors I mentioned above as well as how much abstraction you want. If you use too much blur, the subject becomes mostly faint shapes and various colors blurred together. If you stay in the 1/20th to 1/30th of a second range, you'll get a certain amount of definition in the moving subject.
