An all around disaster, from sales to service. Once they have your money they don't care about you, your car or the brand name they are representing. Paul Montalbano, the owner is unreachable. The GM, Frank Fragomeni is impossible to talk to, he's rude and dismissive. John Ackers, service manager, is a puppet trained to say "no" regardless of the facts.
I'm all about buying American cars, just don't buy them here. The service department will argue with you about whether something is under warranty regardless of the paper and facts in front of them. And I certainly hope you're not expecting a call back from the service manager when you have a question about your car that's being repaired because they're never available.
The best way to get their attention is when GM sends you a letter asking whether or not you were happy with the transaction and/or service. I suggest you hold on to that letter for a few weeks or month in case you might have to return if your car has an issue. Then, evaluate them honestly and send your opinions to GM.