I have not seen anything that indicates they provide a better spark, like-for-like, compared to a copper plug (but nor have I looked for such info really). Everyone I have spoken too says to get coppers for hi-po cars and just change them regularly.
The benefit of iridiums is obviously the longer service intervals which, if you have a car where the plugs a PITA to get too -- and on some cars (Magnas I have heard, for instance) its a real bitch -- they make sense.
I think there is also a certain amount of wank factor. Saying you have iridium or platnum or plugs sounds so much cooler than copper.
LW.