OK...The strongest anti-legalisation I've found here is the "it isn't just about you, it can harm other people" thing (by the way, I voted yes). I believe fully that we should be able to fuck ourselves up as much as we choose, but obviously when the activity impacts others then it becomes a grey area.
My first point against this would be a case of damage control. If drugs are legal, then the money spent on acquiring them goes to corporations and government; while it might not be entirely nice (think ExxonMobil), then at least to some extent it can be regulated. At the moment, heroin sales go...largely to the Taliban, and a lot of (though not all) hard drugs to some sort of criminal cartel involved in stuff a little worse than getting you high (trafficking women against their will for prostitution, violence etc.). So for this reason, I'd say I'd prefer the money in the hands of people we can regulate, if not necessarily trust.
And, if it's legal, then by definition it's cleaner; you don't get alcohol with anti-freeze in it, and if heroin was legal it would be white and give you mild constipation rather than black and making you a trembling wreck. Evidently, the addiction would still be a problem, but I really feel that with an instituionalised system of dealing with this we'd have less crime resulting from theft to pay for it, especially if prices were lower. It would also be considerably healthier for the addict, and thus I don't think that the burden on hospitals would be dramatically increased to an extent where the tax revenue from the drugs themselves is considerably outstripped. While the consumption of ecstacy and marijuana might jump at legalisation, I doubt that the most serious drugs would; people are, rightly, too wary to shove themselves full of heroin and crack just because it's legal.
For the record, I've done marijuana and ketamine (as well as laughing gas, but that clearly doesn't count). I found it a piece of piss to get hold of, and if I wanted acid or mushrooms then I really doubt I'd have much of a problem. I think the war on drugs is, ultimately, based on a premise of well-meaning but misguided concern, and that a lot of the problems associated with the drug trade - as well as with prostitution, but that's for another day - is due to its illegality.
TL;DR - I enjoy heroin pumped directly into my eyeballs.