Anecdote! -- George H. Smith (author of Atheism: The Case Against God) talks about his experiences with cocaine (generally positive) and heroin (generally negative) in several posts here and several email messages (quoted by another user) here. He used it "on a regular basis for nearly 15 years".

A few quotes:

[...] my years of cocaine use were the most productive, intellectually and financially, of my life.

With cocaine, this problem [of knowing when to stop] is exacerbated by its illegality, which means you have to use a product that has been cut many times -- sometimes with a benign substance like baby laxative but more often with something to give it a punch, especially "crank" (i.e., speed).

To this day I remain convinced that if I had been living in a civilized country and been able to go into a drug store and purchase valium (or something similar) over the counter, I would never have touched heroin.