There is one rule about the speed and focal length. To avoid camera shake i.e. blurry photos you should use shutter speed = 1/focal length. Let suppose you use 300 mm so your speed should be at least 1/(300*1.6) = 1/480s. For crop cameras you should take in consideration crop factor too (which is 1.6 for Canon crop cameras)

With image stabilization you get possibility to shoot with slower speed. For example if your lens have 3 stops stabilization you can shoot (from example above) with 1/(300*1.6)=1/480. 1/480/2/2/2=1/60s (the speeds are just example)

P.S. Of course the above rule depend of how stable you can handle your camera. I personaly have some photos, created at 1/8s (w/o IS) which are in focus, w/o any blur. From other side have other photos at 1/100s (with IS) which have motion blur :)