I'll chime in on what I know about meth... I've got two friends (or had - now acquaintances) who were users, and still very well might relapse from time to time. But I know one of them is quite clean.
Meth, unlike pot, is an upper. The people I've known who got addicted to meth liked a thrill, and never did more than try things like beer and pot. They were (and thankfully are, for the most part) very intelligent, driven people; it seems like, more than anything, uppers and meth in particular does a number on intellectual people due to their desire to perform intellectually. Meth is basically the "ultimate" upper, in that it'll make you feel like your brain is operating in overdrive - and it is, just not the whole thing (thus why tweakers can take large caliber body shots so easily without falling over). Twenty four, 48 hours of uptime is what happens with a single use, but obviously they can stay up for much longer. I believe that is what tends to do the most damage to the meth user's mind - the long periods of sleep and nutrient deprivation, and the raised temperatures resulting from the meth use.
The girl I knew went on one such bender, and that alone nearly killed her. Before the meth, she was 130lb of loveliness; afterwards, you'd probably think she was dead if she were lying down. I saw her recently and, thankfully, she's probably back up to 110lb or so, and she's got the light back into her eyes (a very cheerful girl). I really, really hope she's able to stay off it, because one relapse is most certainly enough to kill a person. Unlike drugs which can give you a "bad trip" and make you lose it, all meth is both the good and bad trip.
Yes, there are those out there who want any sort of mind-altering substance, but there's really no accounting for them. They're so severely gone before they even start that, short of locking them in a convent (and maybe even then), their life is already in 5th heading for the cliff.
One more thing about meth: it is very, very environmentally caustic. It's dangerous to have around, period, due to its easy absorption into skin and inhalation, not to mention the ether and other evil sh*t that goes into making it. It's construction is quite environmentally damaging, too, as the refuse is caustic. Anyway... that's one argument for why meth should be banned, or at the very least prevented from human consumption: having it around will negatively impact others, if not the chemical itself, then the crazy f*cking behavior of the person taking it.