Exhaust systems allow exhaust gas that leaves the engine to be channeled away from the passenger compartment while muffling the engine's sound (via the resonator and the muffler(s)), while cleaning some of the noxious, toxic exhaust emissions (via the catalytic converter).  GM's exhaust system does a good job of both, but GM's priorities include making the car as quiet and as easy / inexpensive to manufacture as possible.  

Aftermarket exhaust systems can improve the car's performance by allowing the exhaust gas to escape in a more efficient way.  Also, and possibly more importantly, an aftermarket exhaust system can give your car a great sound.  Many aftermarket exhaust systems also look great and can even increase fuel efficiency.  

Various parts of the exhaust system that can be upgraded include the mufflers, a cat-back (that is, everything behind the catalytic converter) system, the catalytic converter / downpipe, and the exhaust manifolds / exhaust headers.

I've had a Magnaflow® cat-back exhaust system on my Impala since 2009.  The car sounds great and the performance improvement was definitely nice.  I love the look of a pair of 4" polished stainless-steel exhaust tips sticking out from under my bumper cover.  

I did find, however, that the resonator that Magnaflow shipped with it's cat-back really quieted the exhaust tone.  In fact, it was a little too quiet for my taste.  I replaced the Magnaflow resonator with a piece of straight pipe. This definitely helped.  It did, however, make the interior drone of the car quiet loud when the engine dropped into 4 cylinder mode.  Since I have Displacement On Demand (DoD) / Active Fuel Management (AFM) tuned out, my car doesn't drop into 4 cylinder mode, so this is a non-issue for me.  I used a DiabloSport Predator to perform this tune.

In front of the cat-back, I have a custom downpipe with a Magnaflow high-flow catalytic converter attached to a pair of Doug Thorley stainless steel exhaust headers.