I still use this all the time as a 103 rogue. I've had it for years and I just found out that this can be cast on the run yesterday. I usually use it to tag mobs.
The -60 dexterity debuff will be decent to have on some raids, but the 150dd needs to be increased to 1000dd or so to cover the amount of damage a rogue would be losing by clicking the mask (a rogue at this level of the game is doing a lot more than 150 dps). Only 2 rogues are needed on a raid to keep Assassin's Blood on the target at all times (1 minute duration, 2 minute recast), and any other rogues clicking their masks would be completely redundant since the debuff doesn't stack. They would ALL be losing dps by clicking the mask. If the damage on Assassin's Blood were raised, this problem would be solved, and rogues might actually use this item in experience groups instead of just on select raid mobs. As it currently stands, this effect is highly inferior to the free nukes, dps and heals that other classes get on their Vishimtar quest items. If you're going to recycle an effect from almost 5 expansions ago, at least modernize it!
1 Second cast time, damage dealt in that second = 150. Rogues at this level of the game will be doing 1k+ dps on raid encounters, thats a net loss of ~850 damage, more if the rogue is very well equipped.
If they change one crappy effect to another crappy effect is that supposed to satisfy everyone?
Now admittedly, you are losing far less dps than the original poster assumes. A rogue watching his attacks should be able to time the click during delay and lose little damage if any. But it's still, as the OP opines, quite weak compared to many of the others.
Of note, the druid one offers a 1.0s heal. This is not an ability that druids normally have, in any form. But rogues get as a clicky that that had as a proc five expansions ago? My rogue is an alt and will never see this mask anyway. But if he were my main, I would be miffed about this effect.
Ok, I've changed the effect to be Cleave instead of Ferocity, since a ton of people have given feedback that they would prefer that. I've also changed the spell to be Assassin's Blood on a 2 minute recast. This will go live next patch.