Upon hitting level 60, I specced into Dispersion, despite the consensus among shadowpriests that the talent is, at best, lackluster and, at worst, worthless.
And granted, I am only level 60, so any assessment I make of the talent can be only partial, based on my honor-grinding in AV and my Deathcharger grinding in Stratholme.
Regardless — and sadly — I must echo most shadowpriests when I say of Dispersion, "meh."
Despite Koraa's contention that Dispersion is a PVP talent, it has come in handy far more often in PVE:
1. when I pull aggro, fade + dispersion can be useful to live through Drek/Galv/the Baron. I've heard this poo-pooed as the lazy shadowpriest way of "not getting out of the fire" but yeah, I lived so what…
2. when I'm low on mana, dispersion is (and omfg, how sad is this for the class once responsible for regenerating everyone's mana?!) my best bet at quickly refilling my blue bar.
In PVP, I have cast dispersion and sure, as advertised, it blocks 90% of incoming damage. However, a big-purple-sorta-power-word-shielded-but-silenced priest is a sad thing to behold — and an easy target at that. Dispersion lasts such a short amount of time and does nothing to prevent or remove stuns or slows or fears, nor does it remove DoTs or debuffs. So more often than not, dispersion merely extends the inevitable: my death.
The beta forums have been full of suggestion to "fix" the shadowpriest 51-point talent — add some damage component to it; have it clear stuns; return the health regen component that was removed in early beta; lower its cooldown.
As I'm still leveling, I'll go with the cop-out response and say "the jury is still out" for Khæli. Maybe I need to work on using it more effectively, on timing it better. And maybe if I'm going to assess a so-called PVP talent, I should actually spec for PVP.
But in the end, what makes me saddest about this talent right now is more the "zomg" factor than the theorycrafting one. Dammit, even as a purple ball of dispersed shadow energy, this talent is nowhere near as cool as, say, Starfall, Khree's 51-point talent. Starfall is situational too. But when I cast it, I feel so badass. When I cast Dispersion, I just hope I can GTFO before 10 ret paladins roflestomp me.