I personally don't believe in them as I feel that it would be going against my beliefs to even have a go at using them, but I do know somebody who uses a crystal to 'channel energy' and get answers to 'future happenings' in her life.
The crystal is used to find information, and she uses it daily, to find out what she should expect to happen that day.
I would say though that it does appear that her crystal seems to be around 90% accurate, and she does seem to get the answers that she is looking for!