I picked up a BlueAnt S4 yesterday, and so far it works awesome! It doesn't tie into your car speakers, but it's plenty loud, you can Touch the device or say "BlueAnt, Speak to me." and it will say please say a command.
You can say things like, redial, call back, check battery and things like that.
I think once Siri comes out, all you'll have to say is BlueAnt, Speak to me, and then phone commands(it then launches your phones voice control program) which we can hope is siri.
Worth taking a look. I know someone is selling their brand new one for $75 in the market place