After listening to opium for 15 minutes, I didn't get an effect, except one OFF TOPIC effect. After I was done listening, everybody's voice sounded like a mixed up tape, all squirly and weird.
That made me crazy for a bit until it stopped.
I think that's just from the constant waves you hear. If you listen to it too loud, after you take off the headphones everything will have this machine sounding tone. (sounds like a bunch of random beeps to me).