Obviously the Church admits the efficacy of exorcisms, otherwise she would not have officially appointed exorcists. There would be no other reason for exorcism than demonic possession.
I have not, though I have known people whose mere presence made me quite uncomfortable, there was just something malevolent about them. I wouldn't go so far as to say they were "possessed", but they weren't the kind of people with whom I'd want to spend any time if I could help it.
The various well-known exorcists, such as Fr Ripperger, could tell us quite a bit.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.