Re Iridium Plugs ... no idea! but I do have some evidence that "Platinum" plugs are good.
I have a 99 Fiesta (Daughter's "Learn to drive" car) with 70K miles on the clock.
Plugs last changed at 25K - before I owned it.
It developed a misfire.
Took plugs out - They are "Platinum Plugs" - and on each bendy electrode is a little circular patch (about 1mm across) welded to the place where the spark hits.
That's supposedly where the platinum is.
Except .... 1 of the little patches is missing from plug 3!!! (its fallen off) and the electrode has eroded quite a lot. The other plugs look fine.
I replaced the eroded plug with a new NGK standard plug ... problem gone. I replaced them all anyway cos I'd bought 4 new plugs.
So its not all bollox ... even if I used to think it was.
And another thing ... Never use any plug that says "Made in England" on it (some Champions are made in the USA and they are probably OK).