I use them.
The hardness of iridium allows for a smaller electrode, which is less likely to suffer contaminant buildup. This is a pretty good property in an engine that is often hamstrung by people who don't operate it aggressively enough, as well as poor quality modern fuel, and emissions requirements that just plain back things the fuck up.
Also, it is almost entirely immune to corrosion in terrestrial environments, and is a perfectly acceptable conductor. It will hold a sharp edge longer than other options, and it will stay smoother, furthering the reduction of contaminant buildup.
It's not a magic power pill, but iridium in a plug is certainly a good thing if it's in the budget. I consider it a reliability upgrade in a great engine that's got a nasty habit of running dirty, especially around people who don't maintain it properly.
Anything iridium or above is good enough to light the fuel. It's getting a spark one way or another. Also, electricity has a nasty habit of only arcing from one location... So, this nonsense about plugs with ground electrodes hanging all over them is worthless.
Sparks plugs are all about contamination prevention, service life, and consistent, predictable performance. That's why I use iridium electrodes.
I also gap the damned things properly. Gap your fucking spark plugs, people. Use the factory guidelines, and adjust as required for any changes you've made (such as swapping plugs), and take the preferences of your particular engine into account.
You don't need iridium plugs. Run what you want. Just avoid bullshit. Go for copper, properly chosen, and with no fancy tricks. Gap them properly, and you're good.
If you want to take that next step, it's up to you.
Cheers,
Kennith