Here's my take... Hummer has no practical purpose anymore and should be jettisoned as soon as possible. And as much as I hate to say it, GMC is redundant, at least in the consumer market. GM should make GMC commercial medium duty only, and let Chevy tackle Ford, Dodge, Toyota, and the new Ram based Nissan full size.
Buick has a well defined market here, and it is the number one brand in China. There's no good reason to cut Buick (although, way back when, I think they should have cut Buick and kept Oldsmobile).
Pontiac makes some great cars - Solstice and G8 come to mind, but the Solstice also exists as the Saturn Sky/Opel GT, and the G8 would make a great Imapala/Caprice. The G5 is a warmed over Cobalt, and the G6 exists as the Malibu and Saturn Aura. I had a 2000 Grand Prix and I loved it. My grandfather had a 67 GTO. We have history with Pontiac, but given the above, the brand has become redundant, and if GM is to survive, I think Pontiac has to go.
Saturn, for now, seems to have a place as the American arm of Opel. However, they need to adopt a traditional sales model. The one thing that kept me from buying a Saturn is the you pay sticker price policy. Actually, ditch the Saturn name and just sell the cars as Opels here in the US. That's even better.
I think Saab will be ok, once they get a few decent models out. The 9-5 is old and the 9-7x is a warmed over Trailblazer and should be put out to pasture with its stablemates.
It goes without saying that Chevrolet and Cadillac should be kept.
Just my 15 cents...
