1000 AP proc same proc rate as all wep enchants since mongoose.
The premier enchant for all melee dps. It's *really* powerful right now. As a mutilate rogue, in full bloodthirsty gear with 359 weapons & thrown, I have just over 8k AP (slightly more in pve gear). This proc gives 1000, or a 12.5% increase. Always refresh rupture or whatever dots your class might have towars the end of a proc. Trust me, you will notice the damage increase.
The effect stacks if you have it on both weapons. The proc rate is the same as every other proc enchant since mongoose, and over time if you're constantly in combat you should see roughly 20-25% uptime. However, this doesn't proc off of spells, so you will only ever get 1 proc per weapon, unlike Hurricane, which can have 3 buffs up at a time if you're lucky. But that shouldn't factor in your decision on whether to get this enchant or not; 1000 AP is just that much better than 450 haste.