Bleed effect stacks everytime you hit an enemy so id say it is usefull against all opponents.
Yes, this is the common argument, but there are 2 problems against arguing for this skill:
1. Stacking bleeds is great, but stacking means repeatedly hitting your enemy, the bulk should be done by the strike, not by the bleed.
2. Bleeding is a damage/time effect. You have to leave your enemy "on the board" to see a noticable effect from it, this is the opposite of what you normally want to do.
Useful- yes I suppose- exactly what quantifies useful? hard to say.
I make the argument bleed damage is best for a axe and shield build. Something with very good defense and relatively limited offense.