Coming from an iPhone 4s, S-voice is weak! I tell Siri to call my wife, Siri asks, who is your wife, saves it and makes the call. I went as far as setting the relationship in contacts - Spouse - wife on the GS3 and S-Voice can't get it. I had to rename my wife "First - Wife - Last". Pretty lame, but it works.
I tell Siri to remind to do xxx when I get home, as I pull in the driveway an alert goes off. I tell S-Voice the same command and nothing really happens.
Mind you, I really like the GS3, but I do miss the wit and intelligence of Siri.