I've looked into these units before and probably will buy one eventually, but as far as I have ever read on the forums these units do not offer audio streaming support.
I use my for audio streaming all the time. That's the reason I bought it, actually....my car had the stock bluetooth, but I would much rather have the streaming audio. I haven't had any complaints regarding phone call clarity, nobody seems to have noticed that it isn't the OEM quality level...but then I don't use that much, either. The streaming audio I have no complaints about, either, it's certainly no worse than any other vehicle I've been in which supported this.
The factory connections used for calls is only mono. There is a 3.5mm jack on the unit that is used for stereo audio output. You would need to hook that up to either the front or rear aux input.
Or, if you have XM, you could splice into the wiring for that. I went this route since I don't have an XM subscription and didn't want to run wires to the front of the car. The XM box (if equipped) is right next to the OnStar box, so it's a pretty convenient location.
And as someone previously mentioned, the support from Chris is phenomenal. I bought my unit used, and it was an older version which didn't support audio streaming, I had a few e-mails with Chris about it, and his responses were always prompt and clearly answered my questions. Sent the unit back in and had him upgrade it, been working great ever since.