In a nutshell, synthetic amphetamine. It's currently enjoying the same quasi-legal status that MDMA (ecstasy) was back in the 80s, as the manufactures can tweak the formula just enough to make it a different substance every time one gets prohibited by The Man. Eventually they'll have to adulterate it to the point that it won't be fun anymore, and the tweakers will lose interest. However, now that some people have a taste for it, a clandestine market for it will develop.
What happened when X was scheduled in '85 was that people started selling a similar analogue called Eve, which wasn't quite as popular. Eve got banned, and subsequent attempts to create another failed. At that point people figured it was just easier to keep selling X illegally. The same thing's also going on with synthetic pot.
The problem with this, and the fake pot for that matter, is that it's a totally fly by night thing. MDMA and methamphetamine were both pharmaceutical products. People knew how to take it and what it would do. What we saw with fake-bake was that it just got thrown into the market with nothing more than some marketing print. People assumed it was just like pot, loaded up big giant bowls of it not realizing that it's ridiculously potent and wound up in the hospital because they thought they're overdosing (Demi Moore). Same with the bath salts. People are generally better off with good, old fashioned narcotics with massive history, than experimenting with stuff they don't understand because it's new and legal. Another interesting side effect of the war on drugs.