Its mainly becuz they usually cost a lot of money and they dont help you out all that much and over time you could have just gotten the real thing and had far more gains. Thats why a lot of the guys dont have much good to say.
It's just zinc and magnesium and a whole lotta hype.
Get DX7 from www.proteinfactory.com It's a complete vit/min along with other co -factors and has the same zinc at the same dosage as ZMA at a lesser cost.