I tried the NKG iridium plugs last year and didn't see any change in idle of MPG. They are not worth it.
Stick to the copper plugs and change them regularly. It's cheaper and works just the same.
Edit: here's the thread
about the iridiums and my brief comments on any percieved benefits.
For less than $10.00 a year, you can swap in copper plugs every spring and get better results over a 6 year period versus spending ~$60.00 for a single set of spark plugs that will get you 2 years of service before needing to be replaced.