I was about to start a new thread when I found this... 3 sets of end links in 30k miles (all replaced under warranty) was too much, and the 4th set are now knocking (no, no mis-diagnosis, as mentioned elsewhere in this thread- they just suck). This weekend I'd had enough and went with Autozone's Duralast links (lifetime warranty!) for a 2010 Cobalt SS. All metal, slightly shorter (which should be better on lowered car), much nicer, 15 minutes of work.
I'm not an engineer, but I've been building and running cars in Solo I, II, track days and drags for about 30 years- so I do have a feel for what works. The change in feel (besides eliminating the thunking (again)) is pretty noticeable. Quick transients are a bit more "snappy", and it takes a much nicer "set" in long sweepers. The plastic links are pretty flimsy when you hold them, and my guess is there's a significant amount of flexing going on when they're put to work.
Cheap, worthwhile upgrade, IMO. Now if I could just get a bit more rear bar and shocks valved a bit more aggressively...
-mike