A GHB substitute. Also known as BAD, from the acronym, not from the effects. 1,4-Butanediol has a oily taste which clings to the tounge, like having motor oil or wood polish get into every crevice between your tastebuds. It's not entirely pleasant, but in an different way to GHB so you might find it more bearable.
The effects are OK, but real GHB is much nicer. 1,4-Butanediol takes longer to kick in, and when it does it feels more like a sedative. GHB is lighter, happier and bouncier. I wouldn't bother with BAD.