Any hunter who ever does any PvE should definitely have this addon.
Essentially, what this addon does is allow you to view, at a glance, which buffs are covered or missing in a group.
RaidChecklist provides a list of every potential buff. When you access the buff list, it will show you which buffs are covered or missing by coloring the buff names in green or gray. When you mouse over the various buffs (and debuffs), a tooltip will pop up telling you what classes & specs can provide the buff--and, if it's currently covered, which player is providing it.
Some of the buffs and debuffs have a flag icon beside them which can be clicked for even more information, detailing exactly what talents in the classes in question provide the player with the ability to use the buff.
|Click to see a bigger image of this addon in action!|
So in practice, you go into a dungeon or raid with your PvE pet set--let's say, a 5% crit pet (Devilsaur/Wolf), an Agi/Str pet (Cat/Spirit Beast), a Bloodlust/Heroism pet (Core Hound) and an ArP pet (Raptor). You can then pop open Raid Checklist using the (moveable) icon it places on your screen. RaidChecklist will tell you what buffs are missing; let's say a warrior is providing both the cat and wolf buffs, and you have a mage for Time Warp (BL/Hero). Then you know you can bust out your Raptor pet to provide the strong Armor Penetration debuff to your raids' targets. The addon will even tell you which hunter pets can provide each buff, so if you're in a raid missing a particular buff, you could go grab the pet in question before it starts!
It makes being useful to your raid very simple and easy, and far less time-consuming then trying to sort the buffs "by eye."