I picked up the schematic but I never actually made one.  Then when I unlearned engineering I lost all my schematics so I never got the chance to try it out.  According to Thottbot they are good for picking up the occasional mote here and there while you are out questing.  I was mainly interested because you need primal water to make frost grenades, although it seems mote extractors are only good for mana, shadow and life.

And don't worry, I will only use my mote extractor to bring freedom and democracy to nations suffering from tyrannical dictators.