Flurry

Stance. For 5 seconds, your attack rate is increased by 33%, but you deal 25% less damage.

Stance. (5 seconds). You attack 33% faster. You do 25% less damage.

[edit] Acquisition

Very useful for Assassins, as most of their damage comes from attack skill bonus damage, which isn't reduced by Flurry.
Most common stance for IW mesmers, since Illusionary Weaponry ignores Flurry's damage reduction.

Anomaly.

The descriptions fail to state that the 25% damage reduction only applies to attacks.