I personally am against instant replay for baseball. I think part of baseball is the human aspect provided by the umpires. It could also be the start of a slippery slope that entirely computerizes the strikezone.
I could live with using it for homeruns and foulballs when the umpires are simply not in position to make the correct call.
in general I feel as though the umpires in MLB do a wonderful job and are 99% correct in the calls they make.