when my warrior is using a 2hb, it adds around 8% to the amounts he hits for. You can tell by looking at the crit messages, it will say scores a critical hit! (1700) but then the actual hit message that follows will say, hits a xx mob for 1850.
Without this spell, the crit messages and the hit messages are exactly the same, or within 1 point of each other.
Pretty nice spell actually, the 8% hits add up, and if you have more than one blunt using melee would really make sense to use it.