It's easy to use since you can go invisible and just run up behind an enemy. It's pretty damaging (I can't give you any specific figures, unforunately) and, like with all dagger attacks, you hit twice per attack. The problem with it is that considering you have to set it up and it's a kind of one-shot thing (until Invisibility comes off cooldown, at least) it's not THAT great. I've never seen it take off half of an enemy's HP, let alone kill them instantly as you may expect (going off DnD and other similar RPGs - even Halo, for that matter).
That said, my thief isn't dedicated to it, she only has 1 point in one-handed, so maybe if you really go to town with that and get whatever the maximum one-handed damage bonus is...
