I have an '05 STS, and the voice recognition system is pretty useless. And yeah, the dealer -- two of them -- said that the problem was with me, not with the car.

I took it in 4 or 5 times, and finally gave up.
I stopped using it for safety reasons. I don't remember what command I was trying to enter, but it gave me a very unexpected result. I wound up switching the seat / mirrors / steering wheel from the settings for Driver 1 to those of Driver 2. Pretty unsettling when this happens while cruising down a California freeway
Looks like they haven't fixed the problem in three years. You'd think that they would build in some ability for the system to train itself to recognize how a person says certain words, and adapt itself accordingly.
Humm ... maybe there is a SYNC aftermarket adapter.