The drop rate of this weapon is somewhere between 1-3%.
As of writing (patch 5.1) this dagger is largely better than many level 90 weapons in the OH of a rogue. The proc has no ICD and an estimated 5% proc chance. Combined with a powerful MH weapon the proc does some serious damage, especially as assassination as poison ticks crit. With some combat dummy tests, I've discovered that being hit capped is required for a DPS gain, also the weapon scales better with gear than most daggers. Making this potentially a good choice come 5.2 once new raid gear is obtained but new daggers are not yet obtained.