Addendum to this:
Drops: Quagmirran (The Slave Pens)
Equip: +37 to damage
Equip: Chance on spell hit to improve haste rating
Cooldown: Not Applicable
It's really hard to say how good this really is, but the +37 to spell damage alone makes it worth equipping if you don't already have some nice stuff.
I actually got this the other day and must say, the proc on this thing is pretty much... fantastic.
Copied from commentor "Locke" on t-bott:
Just got this yesterday, the proc has a 15% proc chance. That means on average every 6th spell will proc it. With back to back shadowbolt spam it will proc on average every 15 seconds, lasting for 6 seconds, giving 25% faster casts. Over time this translates into an average of 7.5% faster casting. 7.5% faster casting equals 7.5% dps increase. Quite nice for one trinket :)
My experience/observation tells me that what Locke is saying above sounds about right. If you have any experience with Arcane Concentration/Clearcasting, it triggers loosely about as often as that does, which is surprisingly often. Also, 7.5% faster casting, as Locke points out, is directly translateable to a 7.5% increase in dps, which is not only quite excellent but also means that it scales with the more spell power you get, which can give it some longer-term viability potentially.
The only downside I can think of is that given the random nature of the proc, sometimes it goes off at odd times ~ like right as you land a killing blow and are leaving combat, or right before you're about to cycle through a burst of instant casts. Still, if it were a useable trinket and you had some control over it, you'd probably be looking at a two minute cooldown for it, which is fine... but not as nice as this.
So yeah, this trinket is definitely recommendable to other mages and warlocks that use shadowbolt a lot (perhaps not quite as nice for warlocks that use a dot, dot, dot, channel cycle primarily).