Today I saw the Hawk-eye technology in action in tennis for the first time, in the game between Federer and Clement. Now players can challenge upto 2 line calls in a set (3 if there is a tie-breaker). I think it is a cool technology given the fact that we get the results right away (less than 10 seconds). And it should also reduce player tantrums on disputable line calls. If you are not happy, challenge the call. Doodh ka doodh, pani ka pani right away. But that also leaves the budding McEnroes going "You cannot be serious" :)
There were talks of using hawkeye for LBWs in cricket. But that one's a bad idea, as unlike tennis, in cricket you have to predict the path of the ball after it hits the pads. And since we still do not know the exact physics behind the swing of the ball, any technique will essentially be non-deterministic. There are just too many variables for it to be an exact system. Also it will introduce unnecessary delays in the game as the third umpire will have to make decisions in this case.
Technology is useful... but we should take care it does not make us stupid.