A crit for spellcasting doubles the damage, less any resistance. The crits don't get any bigger than this, although effects and abilities that increase damage are taken into account. They do occur more frequently, as per the given description.
This ability is also useful for paladins, especially combined with the AA ability Weapon Affinity, which makes their wepaons proc more often. Pallies get a lot of self-buff proc spells, and Spellcasting Fury does affect weapon procs. Also, if you use your DD to Undead spells to pull mobs, a little extra damage right off the bat will help you hold aggro.