Glad you're thinking about trying it out! You'll like it, but as said on the website, hypnotism is not a toy. You need to be careful, and think of what you're doing before you actually do it. If you plan on doing one of the "normal" files, keep in mind that you will get parts of that ponies personality. Also, once you get very deep into it, (about 1 month or so in, depending how you react to hypnosis) you will start feeling, hooves, tail, and a horn or wings if your file character has them. Here's a list of Pros and Cons:
Cons:
Hooflock: Hooflock is what happens when your hand goes into the shape of a hoof, and simply locks there. No, this is not permanent, but it can get somewhat annoying. Happens when first triggered. May stay while triggered.
Inabilities: You will not be able to walk easily, since you think that you're all hooves and such. Happens when triggered.
Pros:
You're a pony: That's what this thing is all about! A visual and physical effect when you get deep into it.
Other:
Personality: This can be overlooked into being a con, depending on what type of personality you have. Say you're normally talkative, and then you choose to listen to fluttershy's file. Try explaining that!
Trigger: A trigger is a set of words you say aloud to activate your "hypnosis" traits.
And, yes. It does feel relaxing!