A Scanner Darkly has already been said, but I suppose it's worth repeating if a joint recommendation gives my ninja and I any more credence. It really is one of the most intriguing movies I've ever seen, and it does have a lot of serious themes about drug abuse while also not coming across as preachy or fear-mongering (that probably has a lot to do with the fact that the author of the original book was himself a drug user). But, of course, you can't put a bunch of druggies in a room together and not have a few funny moments happen, so the movie also has funny moments aplenty, but I won't spoil those for you.