Personally, I never used to believe in all that tripe, and still don't really. I think the people are professional bullshitter's and know all the right button's to push to make their claim's seem true.
However, my grandad died about 3 years ago on an plane, took a heart attack after moving to my aunt's seat ( who he was going on holiday with ). My aunt got a real bad feeling about it, like a destiny type thing and all sort's of crap, just for the fact he had moved onto her seat. It played on her mind for ages. Anyway, when she got beck to work, she was telling some of her workmates about how he died and how she felt. Next day one of her friend's came into work and said his wife wanted to meet my aunt.
When she went to meet her it turned out she was a medium type thing and started telling my aunt about my grandad and other stuff, without asking any questions. My aunt said all she knew was her name after introducing herself. This woman was able to tell my aunt that my grandad was now with his wife Margaret ( my gran ) and also was able to tell her about him having two wedding ring's, something only my mum and aunt knew about. There was also other stuff mentioned but it's more or less stating the same point I'm making, this girl was for real.
My conclussion, there are people out there who can do it, but they are one in a million and on the other hand, there is thousand's or probably million's of fakes, and that's most likely what you'll end up visiting.