I don't buy the idea that it is just purgatory. Sure it is convenient for some things, like pregnancy on the island, plane with all of them dead, etc. But I don't think it meshes with things like Desmond's girl searching for him at end of season 2, and ethan and richard coming to see juliet on the mainland. It would be lame if it was just purgatory.
But what I am wondering now is wtf is up with the natives, has richard not aged since he met young ben, if people can't have kids on the island how does ben have a daughter, who is his daughter's mother... as usual there are tons of questions, and an episode promising to expose secrets of the island really only raised more questions and didn't really answer what people expected. Now we have an invisible unabomber apparently.