Summon Felhunter is a core warlock ability learned at level 29. It allows warlocks to summon a Felhunterminion that remains under their control until dismissed or killed. Felhunters are excellent against casters, as they can remove magic effects, heal via magic, and block casting.
The most intelligent of fel stalkers are selected and trained specially to hunt spellcasters. They look much like a normal fel stalker, except for a golden band placed around one of their horns marking them as hunters. Normal fel stalkers prefer spellcasters, but fel hunters are so well trained that they ignore melee opponents entirely in favor of spellcasters, especially arcane casters. A fel hunter has the same abilities as a fel stalker. It also manifests demonic spell-like abilities - brought out due to its higher intelligence, coupled with the intense training it undergoes to become an elite spellcaster killing machine.
Devour Magic is a powerful tool that can be used both offensively and defensively, since it can be cast on both enemy units and allied units (including yourself and the felhunter). Use it to strip off shields and buffs from enemy players or mobs before beginning to deal damage. Since the cooldown is fairly short you can remove quite a few buffs from your target with ease (and without triggering your own global cooldown).
Devour Magic really shines, however, when used defensively. It works just like a priest's Dispel Magic in terms of what it can remove, with the added bonus that Devour Magic can be cast while the Warlock is feared, silenced, stunned, or otherwise unable to act. This makes it possible for the warlock to remove fears, slows, sheeps, silences, and magical stuns and roots (such as a paladin's Hammer of Justice or a mage's Frost Nova) without popping a PvP trinket, as long as the felhunter itself is not incapacitated or out of range. For this reason, in PvP situations it's a good idea to remove Devour Magic from auto-cast and instead use a macro to cast it on yourself when necessary. Plus, being able to strip off shadow priest's (or other warlock's!) DoTs is a great help at mitigating incoming damage.
In PvP, the felhunter is highly useful against caster classes, but it's much less effective against melee classes.