Having been exposed to a lot of that ilk of GM Vehicles in the last five years or so (Cobalt, G5, Malibu, Impala... Epsilon, Delta, and W-) I think I can inject a thing or two as I am noticing some interesting patterns here. With regards to build quality (things holding up) and especially rattles which are a pet peeve of mine.
Malibu Maxx (2006, Epsilon 1) - 46000mi currently
-Like BK said, the stupid small storage cubby beside the steering wheel. Barely big enough to hold a bluetooth headset and after a while it has trouble staying shut.
-I am very familiar with the "homicidal chipmunk noise" coming from the blower motor at speeds one and two as it is happening on our Malibu right now.
-It's on it's fourth intermediate steering shaft, and a set of front strut mounts already
-The rear skylight never stops rattling
-The headliner isn't strong enough in its fastening to bear the weight of the dome light and so it rattles
-The rear seat frame rattles quite loudly
-Paint is peeling off the radio buttons
-Rear cargo cover rattles loudly (I semi-fixed this one using cloth tape along the mounting tracks)
-Basically a pain in the ass to drive, so many different rattles all going at the same time that it's actually an aggravating experience that I avoid if at all possible. I don't understand how my mom is able to live with it.
G5 and Cobalt (2007 and 2008, Delta 1) - 35000mi before sold
-Once again like BK said the "satin metallic" paint on the window switch bezel
-Like blu mentioned, they both had the steering column rattle which is even more annoying in cold weather when it's louder and more frequent due to the condition of the roads
-Passenger door panel loosened up and creaked when any weight, even just as an armrest during driving, was put on it
-The window switch on the left rear door of BOTH cars would intermittently fail and could only be fixed by slamming the door shut from the inside with my finger putting the switch in the window-down position
-Like blu mentioned about the HHR the shifter mechanism (in this case an Automatic) was really stiff in cold weather (and squeaky/creaky)
-Everything did feel like it had loosened up over time.
Impala (2007 and 2009, W-Body) - just under 60000mi on 2007, just over 32000mi on 2009
-Compared to the other two these cars are an absolute treat as they have both are almost completely problem free
-Both cars have had the rotors warp a bit so that under higher speed or downhill braking you get a little feedback. Only a matter of turning the rotors occasionally so not a big deal.
-Neither cars had the all too common clunk noise although my 2007 is developing a very slight popping/tapping noise/sensation from the steering column during low speed driving & turning that I suspect is I-Shaft related. I'm taking it in on Monday to see if I can get it covered on my GMPP before it expires.
-Both cars have been pretty much rattle free. The only two consistent problems have been on my 2007, one is the trim between the window and the dashpad rattling, but I figured out that is happening because the cheaper plastic expands and contracts; when it expands is rubs against the A-Pillar trim making that noise. I put some felt tape around the bottoms of the A-Pillar trim and the friction problem went away. The other one was a minor rattle coming from the door lock assembly which loosened up in the driver's side door over time. They tightened it three years ago and the problem went away but now it seems to have loosened up again. When the warranty expires I'm going to take the door panel off and tighten it again, and replace the clips that hold the panel in- that should take care of that.
On the whole though I'd say my brother's 2003 Regal has held up better to nine years of service than the Malibu has to six years of service and much fewer miles. The Regal probably needs an I-Shaft at some point as it is clunking, and potentially some new struts/mounts as the current ones are showing their age, but that's to be expected at this point. Beyond that the car is rattle and noise free, tight as a drum considering its age. None of the little nitpicky issues the Malibu has had. There is seriously no excuse for how horrible every part of that car has worn. A lot of Epsilon 1 have the same problems all over the place and as a result I don't think I could recommend a G6, Aura, or both Epsilon 1 generations of the Malibu to anyone that I didn't wish ill upon. I'll swear by the W-Body as quite honestly it's proven itself to be on the whole a much more durable car if a more chintzy one in the materials department than the Malibu.
For example, the Aura has an annoying and rather useless pop-out trinket bin below the dimmer switch.
Other than that, all of the soft-touch interior paint on high-traffic areas like the door pulls is coming off,
the A/C blower chatters like a homicidal chipmunk on speeds one and two. The car only has less than 75,000 miles.
It has the Delta rattle in the steering column.
and when cold, the shifter is horribly stiff and creaky (I'm thinking maybe somebody spilled a sugary drink into the shifter at some point, but then again, when the interior is hot from sitting in the sun, it works smoothly and quietly like it should, so it's more likely just a junky mechanism).
Plus, the whole car is a cacophony of looseness and rattles.
This after first coming out of a 2011 Impala that was quiet and smooth, but had the W-body clunk in the steering.