I became a believer at my high school graduation party. They had a hypnotist and a handful of students participated while the rest of us in the small room (maybe 50 of us) watched. He did a bunch of stuff that I think most people easily could have faked (and I have no doubt that at least a few were), but then he had them go on a roller coaster. He had them imagine they were about a mile into the sky, and as they started down, you could see one girl starting to get sick. At one point on the descent, he "placed" a gap in the track, and right as they were approaching it, the girl just vomited all over the floor.

So yes, I believe that hypnosis works, at least to some extent.