Speaking as someone who has been on the exorcised side, I can say that it's very unnerving and frightening. I do believe in evil spirits (call me crazy), and I've personally had a run-in with one during my senior year of high school (and, no, I do not have a chemical imbalance or psychological problems. Never have).
Take what you will from it, but here's my experience: I came home on a Wednesday evening after a really bad day at school on top of months of stress, discontentment with life, and family problems. I had had enough, so I went into the family livingroom to scream it out (Mom and Pop were not home at the time). Not long after I did, it felt like someone was pulling my face upwards toward the ceiling, then something went down my throat, and I immediately started dry-heaving. Next thing I know, I'm convulsing on the floor for an hour, screaming "Get out!" I did manage to call my parents somehow, and they rushed home from church along with my grandparents and the assistant pastor. Dad and Mom tried to pin me onto the ground to get me to calm down, and it was all Dad could do to hold me there (he's a 12 year veteran, for perspective, and I can barely lift 70 pounds). Mom prayed intensely for me, trying to exorcise the thing while my head thrashed from side to side like someone was making me motion "No! No!" Finally, after an episode of glossolalia from Mother, I felt something leave in a loud scream, and she vomited onto the floor. I was dazed once it was over, but I remember everything vividly, and the carpet smelled like half-digested bagel for weeks.
Some individuals do indeed have chemical imbalances or brain issues (I have 2 mentally handicapped cousins), but the reason for people acting so crazy during an exoricsm isn't mental illness, it's something being pulled out of them that doesn't wish to leave.