I am absolutely opposed to instant replay in baseball. The umps are a huge part of the game, and they players know that, so there is a certain amount of respect on the field. You'll often see the umps gather and discuss a call, and many times, will change it.
Plus, if umps didn't blow a call every once in a while, what would Lou Pinella do
? Sit on the bench and fill out the lineup card? There is something to be said for a manager who argues a call and gets thrown out of the game, and, as a result, fires up his team and fans.
As for automating balls and strikes, ridiculous. Every umpire has a slightly different strike zone, and it is up to the pitcher to figure it out and pitch accordingly. It is also true that a pitcher like Greg Maddox will have a different, wider strike zone than an up and comer. Why? Because he's earned it, and is known for throwing to the glove. It's also the reason you may see a pitcher throw what appears to be a strike, but doesn't come near the catcher's target, and thus gets called a ball.
Baseball has too many subtleties to be ruined by instant replay.