The stock rotors are junk. The rears on my 04 Impy were completely pitted and grooved at 56k miles, so I ended up replacing all 4 of them at that time. I bought Raybestos PG Plus rotors, and brake pads front and rear and spent ~ $325-ish.
Raybestos PG Plus rotors are pretty good, but I really recommend Raybestos Advanced Technology Pads and Rotors. They are just about the best you can get. Ask them... I know at my store we don't keep a bunch of them in stock but will gladly special order them. They ARE pretty expensive, but if quality is what you want, then you gotta pay the price. Impalas are one of the few vehicles they make the AT pads and rotors for. Check your local auto part store... we actually do keep sets for the Impala in stock cuz the local police use them.