Says otherwise on wikipedia... i've never heard raving reviews about it, honestly.
Scientific studies
A 1999 study was undertaken on NCAA Football players during an 8 week spring training program. The control group was told to cease taking any nutritional supplements. Those who took the ZMA tablets showed greater increases in muscle strength[citation needed]. This study was funded by SNAC Systems Inc. (the patent holders) and one of the study's authors (Victor Conte) has equity in this company.
In 2004, a study funded by a research grant from Cytodyne (another supplement producing company) with 42 resistance trained males showed that ZMA supplementation had no significant effects on total and free testosterone, IGF-1, growth hormone, cortisol, the ratio of cortisol to testosterone, or muscle and liver enzymes in response to training. No significant effects were observed in changes in strength, upper or lower body muscle endurance, or anaerobic sprint capacity [3].
In another study done in 2006, a team of German scientists conducted a study on the effect of ZMA and testosterone levels in the body[4]. The result showed an increase in zinc secretions in urine making it much darker like blood, but no effect on the level of testosterone in the body.