Spark plugs are a consumable product that needs to be replaced occasionally.  My shop manual asserts that the spark plugs should be changed every 100,000 miles (for platinum-type plugs) and the wires should be checked every 30,000 miles.

Getting at the spark plug wires is a mild inconvenience, particularly for the rear back.  Changing the rear spark plugs is a substantial pain in the backside, particularly with the rear bank, as it can be difficult to reach the plugs with the appropriate socket / ratchet / extension you may be using.  I found that when changing my spark plugs, I needed to use different combinations of a 1.5" long extension, a 3" long extension, and a universal joint depending on which plug was being worked on.  I also needed a torque wrench that measures inch-pounds (the spark plugs need to be torqued to 132 inch-pounds) and a wire-type spark plug gap checker (the spark plugs need to be gapped to 0.040").

I changed my Impala SS's spark plugs to Iridium IX® plugs from NGK® and swapped my spark plug wires out to a set of NGK RC-GMX107 wires in summer of 2011.  These plugs run at one heat range colder.  A variety of people that I talked to recommend these plugs for modified engines or forced induction applications.  They're an improvement over the OEM spark plugs as the OEM spark plugs tend to knock much more.  (If you log data with a DiabloSport Predator, DiabloSport Trinity, or other such device, you'll be able to see the knock retard climb as the RPMs increase.)  I immediately noticed the increased performance and I'm very pleased to put the increased power to good use.