I'd really like to know what game you're playing.
Dispersion does not work on pretty much anything in Ulduar you'd want to use it for. This includes Saronite Vapor, Stone Grip, Slag Pot, Constrictor Tentacles, Hodir's lightning cone, or Tantrum. Maybe it's different for the EU version?
It's completely worthless in PvE. I've yet to use it for mana in Ulduar, aside from during Yogg, and even then I probably don't need to use it.
It's decent for PvP, but having a purely PvP 51 point talent in a tree that isn't very PvP viable is kind of dumb.