I've only used it for making certain vaguely explosive toys, haha. Crush it up, drop it into a 3 liter bottle, pour water in, cap it, and run like hell. Do not shake it. I did that to try to speed up the detonation and it went off in my hand. Entire thing went numb. It was kinda cool, but definitely very stupid. Here's a video of me throwing one back when I was in high school...
http://www.youtube.com/watch?v=g6tbHKuXybw
Also, if you're looking at using it to keep things cool, it's definitely good for that. My neighbor used to have steaks shipped to his house that were always frozen despite it being in the middle of the summer because the steak company put dry ice into the container. I was always confused when I was little as to why the boxes were so light, but that's why.