I've never seen one, nor do I believe they're real. The mind is a powerful thing. Whenever someone who means a lot to an individual passes, whether it be a loved one or arch-nemesis, it's understandable that they will still have a presence in their lives, even though they're no longer alive. Any 'ghosts' the remaining individual thinks they see, though; nope. Figment of their imagination, if you ask me.
Never seen but yes. Are way too many people who have experienced the paranormal. Infact im pretty sure in say 50 years it would officially accepted and proven by science even that this is a real thing.
Used to, not so much anymore when I realized it was sleep paralysis. It was pretty damn frightening at first, but since my sleeping schedule has returned to normal, I haven't had an episode in a while.