I assume drug users can be divided into 3 or so categories.
People who used substance X once or a few times and didn’t like it
People who use substance X but generally aren’t addicted
People who are addicted to substance X
So what are the ratios of people when it comes to things like meth, heroin, snorted coke, alcohol, crack, nicotine, caffeine, etc?
I heard Malcolm Gladwell claim that only about 1% of people who try serious drugs end up addicted. Is that about right?
Well it depends what you count as “serious” - what you count as “addicted” and what you count as “try”. [not that if you define all these I’d be able to answer 
]
Many drugs are only serious, because what people will do to have to get them.
You can view some image charts using this search, but most don’t give a percent:
http://www.google.com/search?q=drug+dependence+chart
Plenty of people would traditionally be thought of as addicts, but can’t find/afford the drugs - and go through withdrawal many times - as the go thru good and bad times. Some people can be dependant - and not realize it - a first time pill addict - that goes through withdrawal - will often think they have the flu or something.
Purity also plays a part - street drugs vary in purity - I saw a documentary that claimed rates of addiction to meth were highly correlated with the strength of meth available at that time.
I think it is difficult to come up with hard numbers.