I base my opinion on past experiences and knowledge (or lack thereof). I don't know of any ghosts/demons as popularized by society, but that just means I don't know of any. It doesn't mean it's not possible. I haven't seen much of anything that points one way of the other. But anything's possible given the right circumstances.
I would say though, that many of the "posessions" I've seen (documantaries, books, etc) were likely created by the mind of the person involved.
In regards to the priest question - I put no weight in the power of a priest to do anything that anyone else can or can't do. Meaning they don't have any insight, no special powers, or ear to any god.