Improved Seal of the Crusader (3 ranks) increases the critical strike chance of all attacks against a target afflicted by Judgment of the Crusader by 1% per rank. Sanctified Seals also increases the critical strike chance of all melee and spell attacks by 1% and decreases the chance for seals to be dispelled by 33% per point.
Using Seal of Command or Seal of Righteousness after applying Judgement of the Crusader to your target will yield consistently higher damage. Judging these two Seals will not remove the Judgement of the Crusader.
Seal of the Crusader is NOT a good seal for DPS or tanking paladins. Although it increases your attack speed, it makes each hit deals less damage. So its real benefit in damage output is the additional attack power it provided. Obviously, this means this seal doesn't scale with your gear, unlike Seal of Command, Seal of Blood or Seal of Righteousness. Seal of the Crusader is only good if you have extremely low attack power. This is the reason why no experienced paladins with reasonable gear use Seal of the Crusader.
While Seal of the Crusader is mostly useful for its judgement, the seal itself can be helpful when levelling weapon skill, espescially on slow two-handed weapon types.
Certain pieces of gear can increase the power of this judgement, effectively allowing one paladin to use gear specialised in delivering the judgement and another to judge something else, but allowing both paladins (and even priests, with Smite, Holy Nova and Holy Fire) to receive the benefit from the improvements to Judgement of the Crusader. These bonuses also receive bonuses from the talent Improved Seal of the Crusader.