Yeah, restoring 36% of your mana in 6 seconds sucks for pve...noob.  Not to mention the fact that you can use a mod like Outfitter to swap out high intellect but low damage gear so that you can get the most mana from this, then swap back to your preferred set.

I pop this when I'm out of mana.  One minute later I pop Shadowfiend.  One minute later I use Hymn of Hope.  One minute later I cast Dispersion again.  I keep Vampiric Touch's effect up constantly in between.

I haven't had the need to drink any water for this expansion with the exception of pvp encounters (I usually just Shadowfiend or Disperse right before ganking something, but sometimes I set up for a big fight).

Oh, do you ever Tab-DoT?  You know, the ultimate leveling (PVE) technique?  You know how it's difficult to take all that damage from so many mobs...oh wait, no it's not.

if your talking PvE as in raids your an idiot...

1) runnin outta mana no matter what you are is BAD it means your DPS is going down and if something has an enrage timer like oh say Patchwerk then your screwed...though unlikely that one DPS goin down is not usually a big problem it means everyone else has to work much harder

2) Sometimes there just isn't time to get a drink like if an extra pat pulls or someone accidentally pulls a boss (it happens to everyone at least once) at least you won't be OOM at the start of the fight

All in all this is an amazing spell and has next to no downside

This is to make it easier to deal with healing priests in which mana pools are the basis of the duel.  Same goes with mages and other casters.

I'm 77, and earlier today I made a 79 paladin oom.

I'm nearly positive you can. I use it all the time with lifeblood so that I can heal a bit while dispersed. I don't think engineering would be any different.

I wouldn't be suprised if you could use potions, trinkets, consumables and bombs while dispersed, though I haven't tried any of those yet.

You can pretty much use it whenever you want, even when you are between flight paths.