I'll weigh in on this...
Like a lot of other people, I encountered MJ when I was younger. It was around in high school, but generally avoided it because the craze for testing athletes for drugs was starting (as if MJ is a"performance enhancing" in any way for a swimmer-M. Phelps aside).
In college, a friend of a friend was one of those guys who seemed to have all kinds of stuff---sinsemilla, hash, red oil...etc. and a host of delivery systems. He was marginally a student, but ran a lucrative side business. It was here that I sampled some resinous MJ from his private stash. The result was not what I expected after trying some "skunk weed" in high school.
I was relaxed, then tense and pretty paranoid for a time. My mind raced from thing to thing. It really kicked my ADD into ultra-high gear. I was all over the board and impulsive as hell. I didn't give a fuck what happened. I normally never let ANYbody else drive my 4-Runner, but I was in no shape to drive out to the Strip to go bar hopping. I just handed the keys over to a stranger and said, "To the bars, James.' I climbed in the back and trusted him even though he was just as baked.
Subsequent uses were just as unproductive. Honestly, the ADD was magnified so much that it just wasn't a nice experience. Like Ken, I believe that alcohol is more destructive, but the THC in high quality MJ is not to be trifled with either. But..neither is Adderall, Prozac, Paxil, Wellbutrin, Klonopin....and on and on. Ken, if it is ever legalized here...well, I'll cross that bridge when I get to it.