Surely the 'gateway' argument is old hat by now. Strolling Johnny calls on it to make his rather emotive points, but I have yet to see one serious piece of research that backs it up.
The addictive personality argument is trickier. I suspect that I may have one such, being addicted to nicotine and caffeine. Certainly I also like to experiment with psychoactive substances (for which Huxley can probably take more blame than my dope dealer).
Perhaps I should declare my hand here - I've smoked cannabis for more than 20 years - preferring to grow my own rather than subsidise criminals - but have also used many other drugs over that period.
For some time I used rather more cocaine than was good for me - up to two grammes a week in the early 90s (I was introduced to it by a friend who was vehemently anti-smoking and who had never even tried cannabis). The high was certainly enjoyable (and, Strolling Johnny, it is a stimulant very much like caffeine) and under the influence I was able to work and play longer and harder. The downside was, of course, that the 'downtime' became less pleasant - irritability, paranoia and depression are a high price to pay for a short-lived high - so I stopped snorting and haven't had a line for the best part of five years. I still sometimes get offered the odd line, but it simply doesn't appeal.
Crack cocaine I tried two or three times and, oh boy, I can see its appeal. The high is instant and very strong, giving a feeling of invincibility to the smoker. The downside is very similar to that of ordinary cocaine, however, and comes quicker, leaving the user feeling completely wrung out.
I've smoked but never injected heroin. That was the most worrying drug, because it seemed to produce some of the worst things attributed to cannabis, including demotivation, a loss of 'self identity' and a psychological craving, while wrapping one up in a warm feeling of contentment.
My experiences with LSD (which, of all the proscribed drugs, is the one that most researchers agree is non-addictive) were all remarkably positive, with the acid opening the doors of perception in Huxley's phrase. The downside there was that the apparent perception and lucidity were so transient, with the 'man from Porlock' getting his foot in the door before the effects had fully worn off.
All of the above drugs, however, came from different sources. Most dealers at street level in the UK seem to specialise in one drug - you get your grass from one, charlie from another and smack from a third. At no time when I was buying grass and hash was I ever offered a Class A drug, so the keepers of the gateway were certainly missing a trick there!
All of the above, however, is a personal and subjective view. I know of people who have found that cannabis precipitates psychological problems, just as there are others who can't take alcohol or caffeine.
If I have any regrets at all about my own experimentation it is that it has put absurd amounts of money into the hands of criminals and thus I've connived in the exploitation of others. As a result, my own view is that we should not only legalise cannabis but all other proscribed drugs as well. Like some others, I prefer to reserve my intolerance for organised religion!