I grew up in El Cajon, CA - which had a big Hells Angel (Dago Mob) contingent, who were heavily into cooking the stuff and spreading the epidemic - in fact, it was often said that El Cajon was the meth capital of the world in the 80's.
While I've never personally done it, I can say that I had a lot of friends who were tweekers. The problem with meth is that it induces psychosis, especially with people staying up for days on end - while Heroin users usually will commit crimes to get their fix, after they get it, they are usually peaceful and just nod off. Meth however can turn people insanely paranoid and violent.
Both meth and H can be incredibly destructive, but the way I look at it, is if both were legalized, H users would all just be passed out somewhere, while meth users would be the worst of the worst violent criminals, not because of their actions to obtain the drug, but from the drug's effects itself.
I guess like with anything, moderation is the key. A good buddy of mine used meth when he was a teen, and became an incredible guitarist in a very short period of time - staying up all night, practicing, etc. But he was able to take it or leave it, and quit after a year.