^I couldn't agree more, with everything clocksandshit said. I once took a "recreational" dose, I don't remember exactly how much, but it produced very similar effects to those described above. Everything about it was awful. From what I've read about fever induced delirium, it's pretty much what you experience with diphenhydramine. Confused, sick, tired, woozy, moaning and rocking back and forth... Awful. And now I can't even take diphenhydramine for sleep or allergies because I get restless leg syndrome and even at very low doses I still get twinges of the woozy, sick feeling and I just can't do it anymore.
I do not recommend for fun.
As to why we see spiders, I agree with the idea that it's because your brain is hardwired to recognize them as dangerous, and the dph causes your brain to fire off random neurons (or something), and in the general sense of panic and despair that characterizes the diphenhydramine trip, it starts coughing out random images which it then interprets as threatening.