I think the legal / illegal definition makes it hard to have a sensible discussion sometimes (not necessarily here as people are making some good points and no hint of a bun-fight). What I mean is, often everything classed as illegal is lumped in together, so if you have taken one illegal substance you have crossed an invisible line - and the respective effects and risks of, say, MDMA are grouped with heroin, when they are wildly different in terms of harm to individual and society.
There was a very interesting documentary herebased on a scientific study done a couple of years ago which rated the 20 most harmful drugs ... suffice to say the current classification system came under fire for being outdated and not reflective of truly how harmful a drug was. The often-quoted point was made that nicotine and alcohol would probably be class A if introduced now. But in spite of the obvious health risks of both of these people still take them, and it's the same for illegal drugs too I suppose.
I wouldn't necessarily legalise drugs although I do think we need a sensible dialogue about them and to review their classification. Not least because people will take them, let's not pretend they won't and limit our approach to threats of imprisonment. Why not educate people of the risks and protect where possible - so for example allow people to test what they are about to take to check it isn't cut with all sorts of hideous toxins.
FWIW, I have tried a variety of substances in the past - and my experience of how many people have taken things is often group-dependent. I have hung around with groups of people where nobody has, and other groups where it's much more common. Overall I would think that at least half of my friends / acquaintances have at least tried something, and that's a wide variety of ages, backgrounds etc. Nobody has ever had any lasting ill-effects but I accept that is luck rather than judgment.