Spark plugs are all about conducting electricity between a gap to ignite the air/fuel mixture. The better spark you get, the more complete burn you get, and the more power you get out of the fuel you're using (and less emissions, too). To get a better spark, you need to use a good conductor. Copper is one of the best conductors available for spark plugs, and is dirt cheap, though it has pretty short life span. Nickel and platinum aren't that great, and are only used by manufacturers because of their extended life span.
Iridium is the best of both worlds. Even though iridium itself isn't the best conductor, it's extremely hard. It'll last the longest of any spark plug tip material (some are rated up to 100K). Being so hard, the tip can be made to a fraction of the size of a normal one. This creates a very concetrated spark in comparison, which easily negates the lower conductivity of the metal. Iridiums are know to ignite lean mixtures that other plugs can't, which would definitely help fuel efficiency.
I had an 86 Celica GT-S, and it ran horribly on NGK coppers. Tossed in some Denso Iridiums, and it ran like a champ. Not sure what was wrong with the car, but it really liked iridiums. I'm running NGK Iridiums on my current MR2 Turbo, and it likes them just as well.
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