Apparently the latest version is survivable, can take multiple hits before going opaque and includes a head mount system that diffuses the impacts. It is apparently proof against most 5.56 NATO loads, but not 7.62.
Consider a fastball from a world-class pitcher, like Randy Johnson or Roger Clemens in their heyday. Remember the vaporized bird that Johnson hit? His fastest pitch would rack up about 150 Joules of energy - about 1/10th that of a 5.56mm bullet.
Considering a beaning in the batting helmet from Johnson would leave a guy with a concussion, and vaporize a bird, I'm not sure I'd want to trust a helmet would protect me from ringing my bell VERY strongly from 10 times the energy!
Just like a level IIIA (no crash plate) bullet resistant vest - it'll stop a 12GA slug, but you'll break some ribs and basically be out of commission for a week or more. Not dead, not perforated, but in no condition to do much more than lay there in pain...