By scientician (426 – 2·4) on 2009/07/14 (Patch 3.1.3) Report
When glyphed and talented appropriately, Mass Dispel has a 0.5s or less cast time and costs 637 mana at level 80. You need Focused Power, Absolution, and the Glyph of Mass Dispel to get here.
Specing this way pretty much turns you into the ultimate groupwise debuffer, and if there is ever a situation where it is a giant group vs another giant group a few mass dispels will tilt the scales in your favor very quickly. There have been many a turtling AVs that have turned the tide on because I would show up and cast 8 or 9 mass dispels on a combination of ally and horde players. I got rid of dozens of fortitude, spirit, and mark of the wild buffs, along with several dots, hots, a few arcane powers, and even the occasional immunity. Of course, I'd occasionally hit a UA or vampiric touch, but priests who dispel UA dont talk about priests who dispelled UA.
It is feared by many a mage and paladin on the opposite faction because it can remove immunities, but don't let that stop you from... well... dispelling things en-masse. :)