The only real downside to me is that we'd become a magnet for the international stag party scene. It'd be like Amsterdam and we'd have an invasion of party-tourists who abuse the novelty.
We would see a huge reduction in crime and a far safer drug climate. I genuinely don't think that people would descend into a drug-taking frenzy. I'm not convinced it's the illegality of drugs that puts non-takers off. And it doesn't have to send a message that the government approves of drug-taking. God knows, they warn us of the dangers of smoking enough.
The costs to the NHS would easily be outweighed by the extra tax revenue. Source: me.
You're right to say it's the invidual's prerogative. As JS MIll once said, a person should be at liberty to do whatever they like, as long as it doesn't harm anyone else.
And yet, governments ditch that ideal when it comes to drugs and legislate on what they think is good for us, or to protect some fictional, 'agreed' set of morals. Booze, fatty foods, bungee jumping and suicide are all deemed to be fine though.
For more fun reading, see my dissertation: 'The Rise of Legal Moralism and Legal Paternalism in the UK'.
Actually, don't.