I drove plenty of last-generation GM cars with the up/down shifters. The damn thing doesn't work. As stated it doesn't take downshifts as a mandate to physically do anything.
Note that you said "last-generation GM cars with the up/down shifters." The new Malibu uses a completely different transmission (an extra two cogs) and a different programming setup, similar to the one found in the Saturn Aura XR, but I believe tuned a bit more extensively this time around.
The old 4T60E was reluctant to shift up or down, but there is only so much that can be done. I can confirm that the 6T70E is happy to go both up and down when you tell it too, and furthermore, even if you leave it in "D," it will do most things as you'd expect it to do (meaning, shift when it should, both up and down).
===
Rental or not, this does not prevent it from being an otherwise good car thats certainly worth a look if you're in the market for a new family sedan. There are plenty of Camry and Sebring rentals around here as well, not to mention occasional Mazda6s and Hyundai Sonatas. It all depends on what the company likes, and what it wants to spend money on.
Chevrolet will sell some to rental fleets, so what? They won't sell as many as the Camry either. Who cares?
What matters is, simply put, GM is building excellent cars and trucks again, and it has been a long time coming. If the updated G6 is to be better than the Malibu, that is very good indeed. Furthermore, when the Epsilon II cars debut for 2009/2010 (Saab 9-5, Buick LaCrosse, Vauxhall Insignia/Opel Vectra, Saturn Aura), I imagine they to be quite good as well...