I'm not sure if it's similar to heroin, but it has crystal meth in it which is supposed to intensify emotions, or something like that.
Anybody else want to take a stab at this one?
Sure, if you don't mind me going completely from memory on this one. Ecstacy (or MDMA as it's known in the medical world) was used back in the 60s-early 70s in marriage counseling and group therapy. It worked great for a while because when a person is on MDMA they're senses of touch and sight are incredibly enhanced which is why ravers carry around things like glow sticks to watch "tracers"--or a blurring sensation generated by the eyes--eminate from them as they move around. It also creates very euphoric emotions, like UpHereEh mentioned, hence it's use in group therapy back in the day.
Well, people started dying from it (MDMA causes the body's core body temperature to skyrocket, frying the central nervous system) and around '85 I believe MDMA was officially banned. A few years later it became a new club drug and that lead to the illegal "raves" which are essentially excuses for people to try ecstasy and see how close they can get to contracting an STD.
Sidenote: A poster above mentioned baby pacifiers and pacifiers are big with ravers. The reason for this is that people who are on "E" tend to grind their teeth subconsciously and the use of the binkie alleviates that and the headaches it causes from teeth grinding.
If I left anything out here, feel free to correct me.
As a former hippie, who used to go to Grateful Dead shows all during the 80's, I can testify first hand. X is not like heroin, although heroin is sometimes used as a filler to cut the drug. It is a version of MDA, which was popular in the 60's and 70's. The high that one gets from X is a feeling of euphoria, and has hallucinatory properties similar to LSD. I tried it a couple of times around 1990, and have seen a few folks have some baaaaaaaaaad reactions to it. Very dangerous stuff.