(Epistemic status: I'm not a hypnotist, but I have been a hypnosis subject many times. Experiences described here should all be treated as anecdata. I'm not digging up sources and quotes right now, because I need to stop putting off this post. I will try to update later with the relevant scientific literature.)
When I talk about being a hypnosis subject, I run into a lot of misguided assumptions. Some people think that I lose all autonomy. Some believe that it's easy to make someone do something repulsive. Some individuals with a hypnosis kink believe that the full extent of their fantasies can be achieved via a rotating image on their computer screen. Some of these things might be true, for some. Like most subjective experiences, hypnosis is hard to quantify. Attempts have been made, and there are promising results for pain relief. Less common are verifiable results about utter loss of autonomy, brainwashing, memory loss, post-hypnotic triggers, and long-term behavioral changes.
Again: I'm not saying that these things can't or don't happen. The anecdata says that they don't, with the exception of those with hypnosis kinks. I've only done cursory research into the general experiences of that group, but they're unusually adamant that hypnosis can be instant and has far-reaching effects. Like most things in that category, it's remarkably hard to tell if people are roleplaying or honestly relaying their subjective experience. Feel free to chime in if you belong to this group. I'd like to hear your opinions on the possibilities of hypnosis, sans intentional roleplay.
The other thing that makes hypnosis difficult to describe is that it clusters so closely to meditation, subspace (see comments on kink, above), and acting. I was never very good on stage, but getting into character is like Hypnosis Lite. Meditation is also Definitely A Thing, despite being hard to describe. If I recall correctly, there is more scientific literature on the long-term benefits of meditation than the long-term benefits of hypnosis.
Hypnosis is not like being drunk or otherwise intoxicated. It's more like following a guided meditation CD, with a less annoying narrator and fewer ocean sounds. I have a really vivid imagination. If I'm leaving a creepy basement and think "ghosts" or "Voldemort," I induce a sensation of fear and a prickling on the back of my neck. If I think "be sad," I tear up and get goosebumps. In a lot of ways, the success of a hypnosis session depends just as much on my "skill" as a subject as it does on the skill of my hypnotist.
All of my sessions have been with my spouse as the hypnotist. It generally starts with a few small instructions: Sit, lie down, get comfortable. Close your eyes. This, according to the spouse, is to establish a pattern of following instructions. You'll notice that you already have to be comfortable enough to obey someone telling you to lie down and close your eyes. The initial stages of hypnosis are intended to put you in that frame of mind. If you've ever seen a stage hypnotist work, you'll notice that they start by asking for volunteers, to establish consent. They then select a large group (peer pressure) from the most frenetic audience members (enthusiastic consent). The volunteers end up in a situation designed for compliance.
My most common motive for being hypnotized is pain relief. My pain has never been completely gone during these sessions. Instead, I'm aware that pain is happening, but I don't really care. I don't need to flinch away or tense up. The induced ability to relax is helpful all by itself; a lot of my pain is from muscle tension. It takes about five minutes of repetitive instructions to induce any numbness or genuine relaxation. All the while, I'm aware of the fact that I'm being hypnotized. Occasionally I'll think "I could stop if I wanted." Luckily, I don't usually want to.
Numbness can be induced at this stage, too. My spouse links the sensation of numbness to finger taps, or a very specific region of the body: "Your shoulder will begin to feel numb as I tap it." Numbness doesn't persist very long unless the triggers are repeated, but it sets the precedent for other instructions.
The second motive is play. I like altered states of mind, and hypnosis with a trusted partner is a low-risk way of inducing them. Some hallucinations and temporary beliefs are easier than others; I'm apparently quite good at seeing things that aren't there, but terrible at not seeing things that are. I can experience different perceived temperatures, sounds, and can be induced to act as if certain sensations are occurring. I am still aware on some level that I could stop the session whenever I want. Again, though, I don't really care.
My spouse has tried (with my consent) to induce post-hypnotic triggers. The idea is to give me an instruction that I will follow even after coming out of the hypnotic state. I seem to have a very short window for these instructions to stick, and they quickly fade away. If I was undergoing hypnosis regularly, and reinforcing those instructions, I imagine they'd stick longer. It's an experiment I've been interested in, since it might be useful for instilling better habits.
Eventually, I'd like to learn to hypnotize other people, and work on finding some long-term positive effects from repeated sessions. Even if it turns out to be placebomancy, I'd like to find out how useful hypnosis can be.