One critical thing to understand about the Zena is that it must be fitted properly, then adjusted and tightened properly on the body in order to get the most out of it. That might sound like a pain, but fitting is simply a matter of mixing and matching the pieces for a custom fit, and tightening properly is a simple 10 second process a diver follows at the beginning of the dive. Once both of these elements are in place, the Zena should never move or roll on the body. The tank is held firmly in place regardless of the position of the diver.
But like EVERY piece of gear, not one brand or style is for everyone and trying before you buy is critical.
