I'm Portuguese, so let me give a local perspective. It is important to note that Portugal had a very low crime rate to begin with and, in comparison, drug problems smaller than the US. The experience may not translate directly.

Nevertheless, decriminalization of consumption (not traffic), has allowed drug users to reach out for treatment, which led to better statistics on drug use, better approaches to treatment and an overall decrease in hard-core drug use (cocaine, meth and such). Cannabis use is up from then, but that is perhaps an observation bias (better statistics).

The result of lower hard drug use is a lower crime rate. That effect is clear as water. It is now obvious that most crime had drug use as its cause.

Interestingly, there was a social effect that may be hard to reproduce. Hard drugs fell out of fashion. While cannabis is widespread in youth events, hard drug use is socially shunned. Perhaps the visibility of its effects hit the population, perhaps the relative personal cost became obvious, but it is certainly uncool to be a hard drug user.