I am running Denso Iridiums (although my girlfriend paid for them) and I felt a noticeable difference.
To make sure the difference wasn't (just) that I needed new plugs... I removed the Iridiums and installed NGK coppers and I noticed the same difference in reverse. I ran the NGK coppers for 2 weeks and then decided to swap them out for the Iridiums.
Basically, the engine calmed down at idle - smoothed out a bit. Its hard to explain to someone who hasn't felt or noticed the difference for themselves.
Bottom line with a thread like this... One member will say NGK platinium are best, NGK coppers are best, Denso Iridiums are best.
I say this Iridium has a significantly higher melting point than Platinum and you really need not compare them to copper - the contact around the Iridium tip is not shielded by porcelain to protect it during the combustion process like the platinum is because it doesn't need to be. You will not have any build up surrounding the Iridium tip, and it has a longer life span than platinum (go figure). And more importantly... Less energy is required for the spark to make contact with the ground than in platinum or copper.
Good luck with your choice - but read up on the Iridium - the tip is the d a m d e s t looking thing. Looks like a felt tip pen.