It's nice to see the London Mayoral electioneering is taking it's traditionally serious route... Good ol' Boris - you've got to love a comedy buffoon like that. If there was ever a man who had no need for drugs it truly was he...
I've always believed in the legalisation of all drugs. But I would not be naive enough to think that this would be an easy change, where everybody would stop drinking to excess (approx 730 deaths per month in the UK, according to the National Statistics Office - tobacco's UK monthly toll is around 6500) and become a paragon of sensible chemical usage. It wouldn't happen because as is pointed out above, people aren't built to be sensible. There would still be drug deaths, although probably less as the supplies would be clean - people would know exactly what, and how much, they were taking - and there would presumably be a lot more support and advice available for those that choose to use.
What it would change, however, is the criminality associated with prohibitive drug laws. Why do little "gangsta" kids run around stabbing and shooting each other? Because there's a lot of money at stake and a very long supply chain above them who want paying. Buying your substance of choice from a high street shop and paying tax on it takes all of the profit and "glamour" out of the equation. Drugs are generally cheap, and need not cause so much heartache over something as meaningless as money.
Ultimately, I don't think a great deal would change with society. People who enjoy drugs take them - alway have and always will, no matter the legal status. I've yet to meet anyone who has suggested that they would like to take drug X, but won't because it's illegal. I imagine there would be a spike in usage as a few curious types take advantage of the relaxation in the law, and then levels would return to a baseline. Probably pretty near to where it is right now, I would suspect. All that would really change is that we'd live in a more civilised society that doesn't criminalise vast tracts of its populaton on the basis of... well, I'm not even sure what that basis is to be honest.