They were recently changed so that they have to have your name on, and they have to go into your bank account. Then, they take a while to clear like cheques do.
They are safe to accept if you know what you're doing. If you've never seen one before, you won't know if it's fake or not!
Most people do not use them, as other methods are easier. You get charged fees for buying them (but not for receiving them), which works out to be quite expensive