The question is not "are drugs used less now than they were 30 years ago?" Rather, the question is whether drugs are used less than they would be, had we done nothing. If we have constrained drug activity at all - even if you only believe we have made buying illicit drugs slightly less convenient than going to your local Walgreens, then the war on drugs have reduced drug usage.
The question is how much; I believe that the war on drugs, imperfect as it is, has drastically reduced the available drug supply in this country, and kept many potential users from ruining their lives with drugs.
Simple economics will tell us that by increasing the price of something, we reduce the demand for it. Constraining supply through interdiction and enforcement has caused prices to increase, and increased prices have resulted in reduced demand.
Moreover, you could further argue that non-monetary costs: the risk of incarceration and forfeiture, for example, are an additional cost that rational people factor in when they decide to use illegal substances. Most people with something to lose (like most rational, responsible adults, with careers, families, and reputations to worry about) will avoid drugs, for that reason alone. For most people, the non-monetary costs of drug usage are far bigger factor than the actual cost of the drugs themselves.
Of course, the lure of drugs can be compelling, even for people with much to lose, like Rush - or for those with nothing to lose, like those in the inner city, who frequently succomb to the lure of getting high. The answer is not to make drugs more available - That would only exacerbate the problem. We need to both constrain supply through interdiction and enforcement, and reduce demand through the increased price that occurs as a natural function of reduced supply, and by imposing heavy non-monetary costs on use.