I now have tested the 4S with Siri with 4 bluetoothdevices: a 2009 Prius, a Jabra Cruiser, a Dual XHD7714 in dash Bluetooth compatible car radio, and blue-link BTM-318 radio/Bluetooth headset. Contraryto my first experience all of the devices worked OK . Except for the BTM-318, with which Siri does not recognize when the spoken input is complete (ie, silence). I have to take the phone out of my pocket and press the mike button, which kind of defeats the purpose.
But in all cases Voice control was faster and significantly more reliable than Siri at recognizing contacts in my address book. Most of my original problems were due to testing in my garage which is in a no-man’s zonebetween the house wifi and the 3G network. I have noticed that web access was unreliable in this zone with my old 3GS and it’s the same with the 4S. Even with one or two bars on the wifi, the phone keeps trying to use the wifi, rather than switching to the more reliable 3G signal. The result is usually a failure to connect. So the bluetooth had nothing to do with my original problems (except for the BTM-318).
I have to agree with the other commenters that Siri should switch to Voice Control when the web is unavailable or too slow. Handsfree dialing is the essential voice function in my opinion and Voice control did a better job than Siri with the contact list. Kind of a quandry for me since the other functions of Siri are so cool and useful.