I think they are crap, there's always someone pushing the pointer/glass. Strangly though my dad is shit scared of them and he's a man of science.
I dared to ask him once why he had lost his temper at the mention of them his reply was that he, mum and some of their friends had messed around with one, seems the friends with them dad only knew from his latest job. A woman came through with a message for him. Turned out it was an old girlfriend that even mum didn't know, she had died blah, blah, blah. After they checked it out and indeed she had. He had neither seen or heard from her in years.
Checked the story with mum and she told the same thing, said it really shook dad up, even more so seeing as nobody in the room as far as he knew had known the woman.