Z-MassPM is supposed to be better than regular old ZMA, which I've used with stict discipline and attetion to detail before for weeks on end and never noticed a single difference. I'm not saying ZMA doesn't work, because there's scientific evidence that says it does, but I don't think it's worth the money. Another thing worth mentioning is that I read that calcium blocks the absorbtion of either zinc or magnesium (I can't remember which) and that pretty much means you can't have calcium containing foods/supps/vitamins before bed that might ruin your ZMA.
If your gonna buy regular ZMA check out MonsterMaker at bulknutrition.com. It's pretty cheap and they're a great site.