The intense hallucinations mentioned above are brought on by injection. You don't have to do this intravenously, you can just inject it intramuscularly. The trip thus produced is far more intense than anything brought about by mushrooms or LSD.
I did this once, and I had no idea what other world I was in. When I finally came to, I found myself staring at a screen saver. Someone else I was with completely lost his shit, and refused to give us another kid's K. He was extremely paranoid and very difficult to talk to. When he spoke, each word was understandable, but the sentence as a whole was not.
Most people snort K, however. This I have never found to produce hallucinations. It just makes you feel weird, as adequately described above. I'll just add that when I dance on K, I get this sort of out of body experience. It's really cool, but not cool enough to be addictive.