I read tarot pretty frequently, though almost never for myself.
I also have family that was into it. My grandmother had an experience where she said she woke up one night and had this intensely menacing black figure standing over her bed. She ditched the cards and ouiji board the next day.
There's a book called "The Beautiful Side of Evil" that is about this exact thing, and religious or no it's a fascinating read.
Despite these influences and a strongly religious background I still dabble with tarot and mysticism in general. If demons exist I haven't met one yet. That said, it is never wise to try to gain power over anything except the self.
Searching for health, learning for the sake of learning, and surrendering (not to something or someone else- that's submission. Surrender is to let go of the need to control the details of life, and to learn to grok.) followed truly will create spiritual experiences and kinds of healing in others entirely as a side effect.
So like someone said earlier, the true danger appears to be the ego. However, to entirely discount the experiences related by others simply because they aren't ours is foolish. So it may very well be that there is more to this than meets our experience.
I for one am too stupid curious to not genuinely try to find out.