I'm no expert on ZMA bro, I just know a bit about it.
Magnesium: I have very little magnesium in my diet so I supplement with the ZMA, it keeps me regular every morning NO MATTER WHAT I eat, relaxes me before bed and I sleep better.
Zinc: Athletes use more zinc than the average person, people who have more sex the average person use more Zinc, Athletes who have more sex than the average person, need even more
so ZMA takes care of that too.
B6: Is needed for lots of functions, I'm under a bit more stress than usual so nothing wrong with a bit more B6.
It's not a miracle supplement and it won't add 50lbs of rock hard permanent muscle in 2 days without any side effects or your money back, but for $10/month I love what it does: better sleep, more zinc, more b6, improved mood from better sleep, better growth and repair, no constipation, etc...
ZMA is just vitamins/minerals, if vitamins can boost test, then that's great, vitamins and minerals are thoroughly studied and have a long record of safety.
IMO(not backed by studies for now) if ZMA raises your testosterone significantly, then you were deficient, if you notice a sense of well being, then you're getting optimum amounts of those minerals.