The infernal will remain under the warlock's command for one minute. It can tank multiple mobs better than a voidwalker; aside from the quite respectable melee damage it does, it also has a fire aura that will damage all units within melee range that aren't outright immune to fire. However, infernals do not have a taunt ability, so if you do pull aggro they are generally incapable of re-acquiring the attention of a mob. The infernal is especially useful in battlegrounds; you can drop it on a group of players to stun them and deal some AOE damage.
Patch 4.3.0 (29-Nov-2011): Fixed a bug that caused Doomguard and Infernal to benefit dramatically more than intended from Demonology Mastery.
/Patch 4.0.1 (12-Oct-2010): The spell to summon this pet has been removed and replaced with Summon Infernal, with no reagent cost.
Patch 3.3.0 (08-Dec-2009): These pets now innately have Avoidance like all other warlock pets.
/Patch 3.0.2 (14-Oct-2008): Cooldown reduced to 15 minutes, down from 1 hour. Duration reduced to 1 minute, down from 5 minutes+. Health increased by 30%, damage increased by 60%, and armor increased by 30%.