Against some [censored] who has a knife or thinks that he's Mr Ninja-san they are reasonably effective.
A couple of points to remember:
1) Confine your blows to his extremities. That means his arms and legs. NOT HIS HEAD! Your trying to discourage him, not kill him.

2) Batons are, correctly used, none-lethal weapons. Yes you can kill someone with one, but so can anyone with a club. This has the distinct advantage to you of being able to meet an attack with maximum force. A shattered wrist or elbow joint will take the fight out of anyone real quick, and give his associates pause.

3) LEO's (except in the U.K.surprise, surprise) tend to get less upset about baton's than about knives, guns etc.