Bull Rush, like most of the tactical manuevers is something that is useful in some situations but not others.
Bull Rush is useful for forcing foes into environmental hazards. If there's a cliff, a furnace, a pit of lava, a pit trap, etc, Bull Rushing the foe into it is a good idea.
Bull Rush is also useful for forcing foes into tactically poor positions or removing them from tactically advantageous positions. If your foe is holding the doorway to a large room so that your party can only attack him one at a time, you can bull rush him back to make space for your allies to enter the room. In the same way, if your foe is one or two spaces away from being prone and adjacent to your entire party, you can try to bull rush him into that position. The extra damage from all of your party making full attacks instead of move+single attacks may be more than the damage you would gain by making a single attack.
There are a variety of situations where other manuevers (trip, disarm, grapple, etc) would be useful and they are similarly situational (though some, like Overrun, are useful in far fewer situations than others).
A good way of thinking of special manuevers is to think of the old playwright's axiom: if there's a gun over the mantlepiece in act 1, it will go off before the end of the play. Applying that to the gaming situation, if there is a a vat of acid in the battlefield, ask yourself if you can bull rush someone into it or dump it on their head. If there's a cliff, think if you can find a way to make the other guy fall off. If there's a table, think if you can jump on top of it to gain a height bonus. Etc.