Iridium spark plugs, whats the difference?

It burns at an incredibly high temperature, so the tips of the electrodes should last longer than normal plugs (if you look at an old standard plug, it'll appear to have been nibbled away around the spark gap). There aren't any performance-enhancing benefits other than that AFAICS.

On a minor aside, there can't be much iridium on the plugs as the stuff's incredibly rare. I remember once that a specialty chemicals company I used to do business with at on old job (Johnson Matthey, I think; might have been Custom Powders) manage to spill a drum of stuff over their loading area, and it was more economical to dig up the tarmac and wash the iridium off it than it was to write off the stuff!!!

More expensive ?

Honestly, not a clue. I've used cheap standard plugs at £1.50 a piece as well as those posh variants with three or four ground electrodes at £5 or £6, never noticed any particular difference. If you change them at 10,000 miles probably doesn't matter what you choose. I would always go for a well-known brand, though.

Sounds like not worth the money, I usually go for NGK, having previously NOT got on with so many others.

The Bosch super fours are good, which is all I have used recently. If you change every 10K as swedish skoda says your not going to notice much difference.

Iridium cant be that expensive, as JML say there pen sets have iridium nibs, and the pen sets are going for a fiver at poundstretcher!!!

expensive, no difference at all in performance and don't last as long as the ngk's.....don't believe the hype...you could always try snake oil for that extra power!!

I got a set out of curiosity for my Favorit - wouldn't say there was any significant power increase (I mean, you can easily kid yourself you've got a couple of extra HP but it's hard to be sure). They've been in for 20,000 miles, near enough, and still seem fine, though it's a bit disconcerting when you look at them as the centre electrode is really thin and the gap is about double the normal setting - by normal standards they look absolutely buggered!

Iridium cant be that expensive, as JML say there pen sets have iridium nibs, and the pen sets are going for a fiver at poundstretcher!!!

I think it's more to do with how thick (or think) the iridium plating is. Its market price is only about half the price of gold, but then again even cheapo headphones get gold plating on the jack these days...

A true iridium plug should be able to tolerate running hotter than copper or iron ones do. That said, it's only the electrodes that are iridium, not the whole core like on a copper plug, so the only benefit you actually get is that they stay gapped correctly longer.