Dr David Johnson of Wilderness Medical Associates has a nice blog post about the use of tourniquets. I think I posted this sometime back in another thread, but it seems appropriate to post it again.

In most situations severe bleeding should be controlled by direct pressure. Where this is not possible, a tourniquet is indicated. A Mass Casualty Incident like Boston is another case where they could be a lifesaver. One rescuer can't hold direct pressure on multiple victims at the same time. Being able to quickly apply a tourniquet to stop bleeding on one person, so that you can move on to care for others, could save many lives. Likewise, for the military, under fire it is crucial to be able stop bleeding fast, then do proper treatment of the wounded after the shooting stops.