PvE-wise, mages are very lackluster. Warlocks provide better single-spell dps (shadow bolt or immolate/incinerate in comparison to frostbolt or scorch/fireball), better AoE in most cases (seed of corruption vs. blizzard), better mana capabilities, provided the longevity of a healer (life tap vs. mana gems), better multi-target dps (MOAR DOTS). Warlocks, however, are absurdly gear- and spec-dependent in comparison to mages in order to deal not only comparable damage, but damage that will far surpass that of the mages.
Warlocks get owned by melee in pvp, mages can actually do damage to melee before they get owned by melee in pvp.
Edited, Jul 7th 2008 2:07am by wingsofscion