The speaker started out with a history of the subject, and it was fascinating. Back in Ye Olde Days, there was no anaesthetic, so if a doctor needed to amputate your leg then he'd get you to drink as much alcohol as possible, then Big Jim from the mill would hold you down while he sawed away. One doctor then found that he could put people into a mental state where they wouldn't feel pain, which was quite handy. This addresses the question of fakery: if you go up on stage to be hypnotised, you might bounce around pretending to be a rabbit, but you're unlikely to refuse anaesthetic just for a laugh.
Another question is "Can you be hypnotised to do something against your will?" Basically, hypnosis works in two stages. In the first stage, you program yourself to do certain things, then in the second stage you're bound by that programming. So, if you agree to do something in stage 1, it's not really against your will, even if you agree that you will follow all of the hypnotist's future orders without question. It's possible that you could be coerced during stage 1, e.g. with drugs, but it would be unethical for a hypnotist to try this out. That means that if anyone has done it, they're not going to publish their results! My advice is that you're safer in company. For instance, if you're on stage in front of all your friends/colleagues, it's unlikely that the hypnotist will try to take advantage of you, but a private session might be a bit more iffy.
Anyway, after the introductory talk, our speaker then moved on to a practical demonstration. He said that some people are easier to hypnotise than others, but it's not what you might think; rather than "weak willed = easy to manipulate", apparently the better subjects are people with a lot of self-discipline. Admittedly, I'm slightly biased in favour of that theory, for reasons that will become clear.
The speaker started by hypnotising all of us (about 100 people). We all put our hands together, i.e. palms touching and fingers interlaced. He then got us into a relaxed state, and gave us a simple instruction: when he said a particular keyword, we wouldn't be able to get our hands apart until he gave a second keyword. He then brought us back to normal, said the keyword, and we all tried to separate our hands. It was quite entertaining to watch people yanking their hands back and forth. Some people were able to separate their hands, but I really couldn't do it until he gave the second keyword. Based on that, he whittled the group down, and eventually wound up with about six of us on stage at the front. (I'll use pseudonyms to preserve anonymity.)
As I mentioned, hypnosis can be used as an anaesthetic, so he drew a black triangle on the back of Rosie's hand. He then told her (via hypnotic suggestion) that when he said a particular command, she wouldn't be able to feel anything inside that triangle, i.e. the skin would be completely numb. To demonstrate this, he gave her a sewing needle, so that she could prick herself; he said that it wouldn't be ethical for him to start stabbing people! She poked herself gently outside the triangle, and immediately pulled the needle back, saying that it hurt. She then tried inside the triangle, and said that she could feel it, but not very well. He said the keyword again, and this time she couldn't feel anything at all. in fact, she was cheerfully stabbing herself repeatedly inside the triangle, so he had to stop her when blood started coming out.
His next demonstration was less violent. He hypnotised three other boys so that they could only speak (and understand) Martian. I was the interpreter, so I was fluent in Martian and English, and I relayed questions from the audience. It was quite bizarre, because I specifically remember asking Nigel about his family. We spoke complete gibberish to each other, then I turned back to the audience and confidently said "3 brothers, 1 sister, and a dog". This probably wasn't accurate (that would be really spooky), but I honestly believed what I was saying.
Actually, it's slightly more complicated than that, because part of me was completely aware of what was going on, but without having any control. It's hard to explain, but it's a bit like watching a video recording of something you did earlier: you can see that it's you doing those things, but you can't control the actions. That may sound a bit scary, but it wasn't, because it's not as if I was trying to fight for control. Taking another analogy, it's like when your alarm clock goes off in the morning, and you hit the snooze button. You know that you ought to get up, and in theory you'd be perfectly capable of doing it, but you just can't gather the motivation for it. That's the post-hypnotic effect, or at least the way it seemed to me: more like a duvet than a straightjacket.
At the end, the speaker said that he'd give us a reward for helping him, so for the rest of the day whenever we drunk water it would taste like our favorite drink. He passed along some water, and we each said what it tasted like to us; Nigel took a gulp and almost choked on it, saying "Whisky!" in a hoarse voice. I said that it tasted like Coke, but I was lying: it just tasted like water to me. However, I didn't want to let the speaker down, so I thought it was better to pretend, and I maintained this when other people asked me about it. Later that day, I was talking to Rosie in the dining hall, and she asked about the water, so I gave my standard answer: "Yup, it really does taste like Coke." She then said that it hadn't worked for her; everything else had been real, but she made up her drink at the end. Based on the needle bit earlier, there's no doubt in my mind that she was genuinely hypnotised there, so I think we were both in the same situation. I wanted to tell her, but it seemed a bit too late after I'd just lied to her. So, that was a formative experience in making me more truthful.
When I went up to Durham for my undergrad degree, there was a stage hypnotist performing at the Students Union during Freshers' Week, so I went along to watch. He asked whether anyone had been hypnotised before, so I put up my hand (as did a few others), and he impressed the rest of the audience by showing how quickly he could put us under his spell. I think that was a bit cheeky, since he was taking credit for someone else's prior work, but there was no harm done, and I kept to my seat for the rest of the evening. One volunteer went up on stage, and said that he was trying to give up smoking, so the hypnotist had quite an ingenious approach: he persuaded the guy that he'd already given up, a week or so prior to that. Once he was out of the trance, someone else offered him a cigarette and he refused it. Assuming that this was genuine, it's a bit different to my experience, since it's a permanent effect. Personally, I wouldn't want to mess around with my memories, because I rely on them to be accurate, but other people may feel differently.
A few years ago I went along to give blood in central London, and Paul McKenna was there to help people get over their fear of needles via hypnosis. I didn't take part in that, but based on my past experience I'd say that it's quite plausible. So, all in all I'd say that hypnosis can be a powerful tool.