I wouldn't waste your money on it. ZMA has some massive assumptions tied to its claims of increasing testosterone and IGF-1, the biggest one being that athletes have some sort of huge deficiency in zinc and magnesium caused by piss-poor diets or they're somehow excreting these minerals through exercise.
I think it's safe to say that if you're willing to fork over the dough for ZMA then you're probably paying attention to what you eat or at least taking a decent multivitamin. It doesn't matter if it's ZMA or another related supplement.
Supplementing with it will not raise anabolic hormone levels if there's no shortage of these minerals to correct. And then of course there's the premise that a deficit in these minerals does have an impact on T-levels.
That being said, as others have mentioned here ZMA can be a successful (albeit a pricey) sleep aid. This is why it's recommended to be taken before bedtime. Getting a good night's sleep plays a significant role in recuperation and hypertrophy so if you have a problem counting sheep then you may want to give ZMA a try. Personally, I'd go with another effective but much cheaper supplement such as melatonin as a sleep aid.