Well, I've read pretty much all there is to read on BB relating to iridium spark plugs, and I was as skeptical as anyone. Finally, went to the local independent parts retailer for my wife's 2003 Honda Odyssey, and the manager showed me why platinum and iridium plugs would be used instead of standard wire plugs. I bought and installed the iridium just before we were heading on a long trip. Gas mileage improved from 22 to 25. The theory is that the iridium provides a more complete explosion in the cylinder and with the thin wire also reduces stress on the coil.

On our return from the trip, I put them in my 745t. Around town, there is no gas mileage difference. But it does seem to have a little more power and steady acceleration than it did before (close to full throttle). Then took it across the state to a Volvo show, and on the trip noticed about a 1mpg improvement over trips with standard wire plugs.

Yes, they are expensive. Consider the application and possible benefits before installing iridium, but they do seem to come with noticeable/measurable benefits.
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94 944 n/a 260,000 miles; 92 745T 150,000; 03 Odyssey for the wife