there was an interesting article on yaba proliferation through thailand in Ralph magazine (of all places!) several months ago. Seems to be a mix of stimulants as stated above, meth, speed, caffeine, cat, basically everything is chucked in together and mixed up.
Don't go paying big money for these things, as stated previously, they cost virtually nothing in thailand.
Plenty of horror stories about people going schitzo etc, but too much of something that unclean and your obviously gonna do some damage I reckon.