There's a class trait that increases its potency dramatically. I'd link to it, but most sites still have the old, "mistranslated" Shadowstalker trait in its place. It's the level 32 trait. With it, Sneak Attack's potency becomes 300, or 500 when used in front of the target, at which point it becomes astoundingly potent.

Until you get the trait, though, Sneak Attack does feel a tad impotent.

Edit: Found it - http://ffxiv.gamerescape.com/wiki/Enhanced_Sneak_Attack