50% less time between attacks
When all you care about is offense, the Haste Helm is the way to go. Its defense is adequate, but there are better helms out there, and many of them have other useful effects (such as status protection). The Haste Helm offers no additional protection, but it does give you a permanent Haste effect, which will greatly increase the wearer's offense. It's better used on a character who uses a lot of basic attacks or cheap techs, though, because their MP consumption will be increased along with their speed.