This ring is a great help for a blue mage up to 75 for those worried about there blue mage spell modifiers HP being a important one, considering you have 600k. :)
A lot less convenient to carry around on Thief than a pair of agi rings, but at least it'll provide you an additional boost for an evasion build if you wanna carry em. I've got a pair, but I usually wind up using a bomb queen (HP) or rajas (subtle blow) as the agi alternative instead.