With the highest respect to JimmyH, its a work in progress in that it is a device you can purchase commercially and install today, but just expect a high beta-test quality to it.
There are often a few seconds' long delay at the beginning of calls, and about every tenth call or so, the module will unmute the stereo system but leave the phone in bluetooth mode. When that happens you can't just reach for the phone and put it to your ear; you have to manually switch it to handset mode and lose a few seconds of call audio, or switch to handset mode and back to bluetooth mode if you are in a hands-free state. Yesterday, someone asked me to show them the kit, and of course, for the first (and only) time, the kit failed to link up with the phone, so I had to manually restart the bluetooth link.
In my '04 CTS, the voice button still triggers the voice command prompting, but the system no longer recognizes any commands. I'm sure that's because the cabin microphone is rerouted to the BlueSTAR module.
I have a Blackberry Bold, and the phone has an echo cancellation feature. With it set to auto, I've had two callers claim they can hear themselves, but numerous others not make that claim. In fact, one of the callers asked what I did on another occasion to fix the echo problem, but I hadn't done anything.
By and large, I am happy with the kit, and I don't see it going back. I certainly do wish there was a complaint-free kit that does A2DP (streaming music over bluetooth) as well as phone calls, but I am definitely happy with the functionality I have today. Its not everything I've wished for, but it has certainly been highly functional for me.
Jimmy's DIY for installing the Parrot kit is top notch, but you still have to place a display screen, another remote duplicating existing buttons, and hang an external mic somewhere in the cabin. And apparently your stereo gets muted for a second whenever you receive text messages -- I exceed 1,500 messages a month, and would punch the nav screen after a day or two of that. I have to believe there's a setting in the phone's ring profile that fixes the short muting, but I haven't seen anyone try to resolve that issue.
EDIT: You don't actually connect it "to" your existing OnStar module. You disconnect the wiring from the cabin accessories (buttons and mic) and plug them into the BlueSTAR. No sharing. Unless Chris of Costar decides to add voice recognition functionality to the BlueSTAR, I would say its gone for good. Check costartech.com for current pricing. I bought the BlueSTAR PNP and it took about 10 minutes to install.