This happened a few years back:
I hadn't had it for a few years, when my old trippin mate "J" suggested we try this stuff he got of "some customer who wandered into the service station he worked at".
I had misgivvings after several kinda bad experiences, but thought "ahh well, for old time's sake..."
It was the most powerful stuff either of us had ever tried, and we spent a full 4-5 hours watching a cactus in a pot dancing to The Beatles - Sgt Pepper album (among others). I remember at one pount, J and I were laughing so much, I became alarmed that he was going to die from laughter; his face seemed to puff up as his laughing became out of control, and he was going all red. We'd settle down for a few minutes, and his face seemed to "deflate", and go back to it's normal sickly pallor, and then something would start us up laughing again, and the whole cycle would start again.
At one point, I went to take a pee in the front toilet. As I was standing there, the wallpaper which had some geometric design on it, was making barely audible fizzing and popping noises. Over this, I could hear this roaring sort of noise building up behind me. I turned around, and this huge animalistic motorcycle was leaping at me from a poster which was taped to the back of the toilet door. It appeared to be coming right out of the wall at me.
Quite suddenly, everything went bad and scary as hell. I suddenly realised that it being a weeknight, I became preoccupied with the fact that I had to be at work the next day (warped time sense - seemed like it was only 5 minutes until I had to be at work, when in fact it was more like 8 hours). Some "wierd" friends of his had dropped around and I was having a hard time telling if they were saying "wierd" stuff to me, or if I was mis-interpreting what they were saying (BIG communication gulf), and to top it off, one of his wierd friends seemed to be coming on to me (I'm not gay - not that there's anything wrong with that), and it flipped me out so bad, I went to a dark room at the back of his place, and "hid" under a blanket for about 3 hours.
Every time I went to the loo, the roaring motorcycle/animal/creature thingy would be waiting behind me, snarling, and making teeth gnashing noises, and always would leap out at me from the wall poster, no matter how quickly I turned around, or how ready I thought I was.
It wasn't until my brother arrived the next morning on his way to work with a packet of Serepax like some angel from above that I started to straighten out. Needless to say, I called in sick (with all the associated paranoid thoughts such as "I'm sure they'll be able to tell I'm pulling a sickie"), and I wasn't properly straightened out until the next morning after that.
It was hugely draining, what with all the endless pointless discussions about differing points of view and so forth, which seem to have huge meaning at the time, but upon reflection are utterly trivial.
I haven't done it again since then...