I changed the plugs on my '04 V6 LTD last month with the Denso Iridium IKH20 / 5344. My 4R has 62k miles on it. I haven't been driving a whole lot recently so I don't have enough data to show any improvement in mileage, but it does run smoother and is slightly more responsive. I got them for just under $12/plug from sparkplugs.com. I know some of you would think that it's a waste of money, but I think they're worth it if you factor in the 100k change interval. The IKH20 plug has a .4mm center electrode versus the SK20HR11 which has a .7mm center electrode (should last up to 200k). The regular copper ones have 1.1mm center electrodes. There's research data online showing how iridium is far superior to platinum and copper. I'll keep you guys updated as far as mileage differences.

p.s. - sorry I forgot to take pictures of the new plugs b4 I installed them. It was a hot day and I just wanted to get it done quickly.