If you're looking for the typical Wizard type of stuff dishing out huge damage at range the Witchcraft isn't the place to go. So I wouldn't count on it as a primary DPS thing, although Death Punch is pretty awesome once you get to high levels.
Where Witchcraft shines is some amazing utility. Oath of Destruction, Blind, Mass Weakness, Enfeeble, Summon Undead... all very, very strong spells, just not really damage.
If you're going for an archetypal Wizard, I am not sure Witchcraft is always good idea, at least not until you have your damaging spells all lined up and working well. But if you're going for any type of hybrid - in other words, a caster with a weapon - then it almost elevates itself to must-have status.
