If the oil life monitor said 29% left and it only had 6k on that oil change then it was pretty much beat on since the oil was changed (not surprising it is a rental, afterall). That is probably Reason #1 for the poor gas mileage. You shouldn't have to disconnect the battery to reset the mileage setting, you should be able to just push a button (it is a check mark button in our 2010 Equinox).
I drove a 2009 Malibu last spring and was impressed. It looks nice and handles nice. I was very impressed with the transmission and the power was adequate. The interior was nice for its class. My only issue was head room.
I had a 2002 2.4L V6 Impala from 2004-2005. The factory rotors were horrible, only lasted 10k before becoming warped. Also had a manifold gasket leak just after the warranty expired (only 36k powertrain at that time). We got rid of it for our Vibe in 2005. But it did have good power and was comfy. Good gas mileage, too (got 36 on the hwy once, usually 31-33). Just too many issues. I think the 3.8L models would be way better, but at the time those were out of our price range.
As for current GM vehicles, our 2010 Equinox seems well built and we haven't had any "problems" with it yet with almost 15k on it. The 4 cylinder engine is a little underpowered (but it gets over 30mph hwy! - we usually get about 30 at 70 mph with the air on) for its weight, but if you want to drage race you'll be looking in another segment anyway. Nothing else in its class comes close (we drove them all) except the new Sorento, and those reviews are not good re: quality (I was surprised considering the supposed strides in quality Hyundai/Kia has made).
If I was in the market for a family sedan I'd probably look at the Mazda 6, Nissan Altima, Malibu, Fusion, and Sonata. All the other imports seem bland (and not just in the mid size sedan segment). I don't trust anything Chrysler at this point in time except maybe their Ram trucks. I like the GM warranty a lot as well, and like I said with my Nox, GM quality seems much better than even five years ago.