Quite useful, since it gives you first chance to cast spells in sieges (provided that the defending hero doesn't have Artillery).
Recommended for: Heroes with walker-heavy armies
While useful for breaking open the gate quickly, one also has to care to consider that it is the only battle-skill that in non-siege battle situations has absolutely no impact. Therefore, in a multiplayer game it may be better to select a more multi-situation skill upon which to invest. In single-player games, more useful, but also situational, as the AI tends to charge out of the castle if an army demonstrates any significant long-range damage abilities. It is very rare that the AI stays in the castle entirely, and that this skill is entirely necessary. I would take this if I had walker armies, and were offered an alternative skill considerably less useful (ie. eagle eye, learning).