So here's the deal. I could not get the ISimple ISGM535 to work with the Bluestar. Calls in or out would not work, Siri would not work. It streamed stereo quite well, but it was not compatible with the phone, which is the main reason I wanted this. It was compatible with Onstar, but only if you connected your phone directly to the ISGM unit. I'm sending that part back.
Regarding the Bluestar, the install would have taken 20 minutes if I didn't screw with the ISGM, and if I knew where the Onstar unit was. So for those with a 2005 Tahoe or something similar, pull down your glovebox and look up on the right side. The Onstar unit is silver and has four ports populated with either two or more cables. Mine had two with white connectors. I simply unplugged the two connectors, plugged them into the Bluestar, and zip tied it to a nearby post that already had two nifty holes for the ties. Total cost: $299 for the Bluestar, and $5.99 for a pack of six million zip ties.
The phone buttons on the steering wheel and rear view mirror work great for initiating the call. I have an IPhone, so it pulls up Siri. For streaming music, you hit the red button on the mirror, and then start the music on your phone. Again, this is in mono, but the sound is ok. If anyone ever comes up with a good adapter for the GM Class II radio that works, I'll go with it.