I think of Purigen as "the new carbon". It's a similar concept. However, Purigen is suppose to have better qualities than carbon, and it can be regenerated where carbon cannot.
+1 this is exactly right. As Liam said purigen lasts "forever" as long as you keep regenerating it. Carbon will also begin to leech the absorbed nitrates/nitrates/ammonia back INTO the tank water if not changed often. Whereas purigen turns brown and lets you know to change it.
This is great info, will bookmark to show the noobs