Samsung I9300 Galaxy S III review: S to the third - GSMArena.com

"S Voice challenges Siri, isn't quite there yet"

One of the things that made a negative impression was that the speech to text transcription was somewhat worse than Siri (which isn't perfect either). S Voice understood the commands most of the time, though there were cases when it didn't quite catch our words correctly.
This was most prominent when we tried to send a text message with voice dictation only - sometimes it would get as much as half of the sentence wrong.
S Voice also needs some polishing when it comes to removing unnecessary prompts - it asked for GPS to be active when checking the traffic in a city (even though we weren't in that city and it shouldn't matter where we are anyway), it stumbled when there were multiple numbers that an SMS can be sent to (it reverted to regular touchscreen interaction, forgetting we're trying to talk to it) and then it crashed when we tried a question just for fun.