El Mahdi
Muad'Dib of the Anauroch
I always stack the critical damage. I've known some dm's who make it instant death, which happens a lot anyway with stacking crits. When the player's roll critical misses, I usually make them hit an ally. Sometimes I make them drop their weapons. Sometimes I get more creative, especially if they have the unlucky flaw.
I still use the "Critical Hits and Misses" table from Dragon magazine. It incorporates all of those results, and adds a multitude of others. But I think I might incorporate in that double confirmation idea of Hawken's and use it with the chart.