There are several enchantments, enchantments have two ways of doing it, either a full blown one element or a mix of two elements.
Some elements don't combine, such as shadow and lightning/wind.
I'll list enchantments by the degree of usefulness in my opinion.
Shadow steals HP and MP from enemies, it has been nerfed because of some classes with too much multihitting, the amount stolen depends on which attack you used, as two strike attacks will steal much less, but most likely, steals twice. I believe the level of the weapon also matters.
Lightning/Wind(the name varies between people)
Lightning/Wind deals 30% more damage when activated, it also nulls enemies' defense for that one activation, this also means it does full damage if the enemy is knocked down.
Poison and Ice
Poison and Ice both slow down the enemy. Ice is just for slowing down, it's also only usefil in pvp. Posion does some damage every moment, not that much though
Fire does a set amount of damage per moment, but much higher than poison, it's pretty useful when you're against def mobs, or super armor mobs.
Petrification, I never found that useful unless
I never found this useful, well in theory, it's really useful, such as if you petrify Barud when he's going to use laser, or when Mutated elsword is going to use Arme Blade, but that's just wishful thinking.