That's a super tough question. I wouldn't go back and not do drugs, but I'd go back and do them differently.
First off I wouldn't have started with Crystal Meth as the second drug I put in my body (3 weeks after I took my first hit of pot). Meth did teach me to have confidence in myself and I learned to walk up to a stranger and make a friend, but at the same time I did some damage to my body and developed potentially life-time habits that will be very hard to break.
I wouldn't have done DXM. I would never have used a needle. I would never have introduced any one to Heroin, that still haunts my soul.
And I would have gone to a doctor back in late middle-school / early high-school to deal with my anxiety/insomnia/depression issues, rather than learn to self medicate with copious amounts of opiates and benzos. I truly wish I had learned to use benzos responsibly, because a legit script would really make my life a lot more manageable; while my current lifestyle involves me finding them when I can and binging on them when I have them. That, and my 10 year opiate habit makes doctors refuse to prescribe benzos to me.
Pandora's Box man.
You bought the ticket; take the ride.
Etc.
Edit: The only two drugs that I think have had a large and positive impact on my life are Marijuana and DMT. Opiates are the class I can't figure out. When I'm living in my opiate addiction (which is 99% of the time) I am in prison. It may be a gilded cage; but it is still a cage.
As some one said earlier, drugs made me the man I am today, but I would certainly have treated them with more caution and reverence.