I can see why you put it in the controversial section but I don't think it should be a controversial issue but a very important one.
It's a funny sign of the times that it is.
We have lost our sense of the spiritual world. We have become very much embedded in materialism. You can thank science for that and that it has become a religion and philosophy in its own right which has in turn encouraged atheism and scepticism to go on unchecked.
Personally, I have never experienced possession. I am a born again believer in Jesus so I know that the Holy Spirit resides in me and all believers. When you have the Holy Spirit in you then you cannot have any other spirit in you (i.e. demon possession)
Petros2015's analogy is a perfect one. When we don't have Jesus/ Holy Spirit in our car then we are an empty car (in the spiritual realm).
I watched a video the other day that really made me think. The lady who made it said that in the spiritual realm, you have to invite the spirit in. You must open the door and invite them in.
When you become a believer in Jesus Christ then you are inviting the Holy Spirit into you.
Similarly if you play around with the occult or other non-Christian faiths then you are opening the door and inviting another type of spirit, a demon. After all, demons are fallen angels. They were once part of the heavenly host but one third of them rebelled against God.
Jacob saw a stairway whereby angels were travelling between heaven and earth. In the Book of Job, Satan told God that he had been busy moving about on earth and ostensibly "working".
It is interesting that my Pastor (for want of a better term, he runs an online bible study group) believes that age of demons and casting them out has finished.
My experience has been otherwise. I have had experienced something that I would describe as demon oppression. Demon oppression and spiritual warfare are the same thing. I have noticed that I have had more oppression since becoming a Christian (and many people agree on this is the case)
Then again maybe I had it all my life on and off but not been aware of it. We use our own free will to sin but I do believe that demons can tempt us to sin. Remember that Satan tried to tempt Jesus himself!
Personal experiences with demons-
1) I have been demonically oppressed in my sleep a few times. Saying Jesus' name immediately stops it all.
2) My sister's step daughter has seen a cloaked figure sitting on the end of her bed a few times during the night. Personally I think that her preoccupation with Harry Potter and new age stuff has made her vulnerable. It's interesting that the demon was sitting close by. Was it waiting to be invited in?
3) This girl's father works at a stately home. He has seen a "ghost" there with his own eyes. I'm not sure about ghosts but I think they are probably just demons taking human form. I have seen a "ghost" myself a few months ago. This is the first time I have ever seen such a thing in forty years.
I am now coming around to the controversial belief that a lot of things that are now explained away by psychiatry are actually connected to the spiritual realm.
I'm starting to wonder whether extreme personality disorders such as psychopathy are not demon possession.
I know this all sounds crazy but I have spent years researching one particular disorder which is called "narcissism". There are many, many aspects to this disorder that science cannot explain. They cannot explain how it occurs (people are not born with it) and it cannot be treated.
What is incredible about this disorder is the "hive mind". What this means is that two people from opposite corners of the world who have never met each other and are not related can have identical behaviour and even speech patterns. They can't understand how the hive mind works. I think that the really simple answer to it is that these two people are possessed by the same type of demon. Yes, I think these people are possessed.
It isn't foaming at the mouth possession, it's more subtle.
I think there are different types of demons that have different "aptitudes".
I think things like alcoholism are facilitated by demonic oppression. How else do you explain how faith alone can cure alcoholism?
What I would say is that casting out demons is a risky business. I believe it can be done still to this day but I wouldn't know how to go about it as I am a baby Christian. I think you would need to really know what you're doing.
To sum it all up, yes demons are still amongst us. They can oppress us and even possess us but we cannot be possessed by them when we have the Holy Spirit.
We are living in a spiritual battlefield but this should not make you afraid. We know who wins this battle in the end. It is written.
Jesus has already overcome the world. All we need to do is believe in Him.