Before going to the dealership, I confirmed that they would accept GMID. I then went in and tried to negotiate with them. Their best # was about $1500 off MSRP so I then presented the GMID thinking I might get the 1500 in addition to the GMID.
My salesman got a little miffed and claimed that GMID was the best discount I could get and they were not allowed by GM to do anything more. They wouldn't even throw in an ashtray even when I threatened to walk out. I caved.
I didn't believe the story but they weren't giving me another dime above the GMID...
YMMV.