The most obvious benefit is that testers in the open beta will receive a code for a special weapon, done up in gold and silver with turquoise inlays. A contest is also being held for the "best" bug report, with the top prize worth $30 in the unfortunately-named potcash and a special hat. As if that weren't enough, there will be special GM-hosted battle events and question-and-answer sessions with the GMs, offering assistance in learning how to play and how to get the most from the game. Gamepot is throwing quite a bit behind the launch of the game, so if you're looking for a new free-to-play title you may wish to give Fantasy Earth Zero's beta a go.
[ Thanks to Evan for the tip! ]