Here's what Ken Jennings had to say:

Watson does have a big advantage in this regard, since it can knock out a microsecond-precise buzz every single time with little or no variation. Human reflexes can't compete with computer circuits in this regard. But I wouldn't call this unfair ... precise timing just happens to be one thing computers are better at than we humans. It's not like I think Watson should try buzzing in more erratically just to give homo sapiens a chance.

Here's what Wikipedia says:

The Jeopardy! staff used different means to notify Watson and the human players when to buzz, which was critical in many rounds. The humans were notified by a light, which took them tenths of a second to perceive. Watson was notified by an electronic signal and could activate the buzzer within about eight milliseconds. The humans tried to compensate for the perception delay by anticipating the light, but the variation in the anticipation time was generally too great to fall within Watson's response time. Watson did not operate to anticipate the notification signal.