Good additional commentary Tim - thanks. Not shooting crop I neglect to think of those issues.
Yuri, you are correct in stating that the slower shutter speed can increase the possibility of "blur action.".... been there done that, sometimes with very pleasing results, and sometimes not. The flash duration may be as slow as 1/500 - don't really know the Canon spec on that - but that will give the effect of a 1/500 second shutter speed. Of course if there is sufficient ambient light due to your slow shutter speed, or existing lamps, you will get "comet tail" blurring on fast moving limbs, people, etc.
This can result in some very interesting dance floor effects with band lighting, smoke, spotlights, etc.