I've overdosed not intentionally, but unintentionally on THC. I don't remember the amount, but I do remember how I felt. After having taken a bong rip of some good-tasting shatter, I proceeded to smoke half of a fat house joint. With good company, I was soon on my way to eat at an AYCE restaurant (perfect treat!). It wasn't until after the walk there that I noticed everything I was seeing and hearing started to merge. By that, I mean the noises and visuals I was beholding became one big, loud entity (obviously the incoming, intense hallucinations).
Once I made it to the restaurant, there was a somewhat long line to be seated. It wasn't until all the noise became intensified, and with the terrible music, that my head started throbbing; it wasn't a physical throbbing, but more like mental pulsations. I was seeing and hearing everything go along to the beat of my pulse. It got really intense that it gave me an incredible headache, most likely due to all my receptors, visual, olfactory and auditory, being intensified. I soon saw myself sitting on the floor (all the damn seats in the lobby were taken) leaning against the wall, holding my head. I felt nauseated and I almost couldn't move. It became hard to keep track of everything I was doing, like my memory was shot and I could only remember things for a one-minute time frame. It wasn't until I started gorging down food that it started to go down. I was more than glad knowing the dysphoric sensation had subsided.
But yeah, to some, it's fairly unpleasant.