I just changed mine for Denso Iridium, not noticed any difference and I didn't put them in for any difference, just a better quality sparkplug - I heard a lot of rave about them in the past in other cars and thought I'd give them a go.
From what I've heard they melt quickly as well.
They didn't solve my missfire problem but that could be down to the oil pissing out the top of the engine down the sparkplug ports.
If I were to buy more - which I probably will in 6 months - I'll probably stick with the NGK's but a decent set like triple spark or platinum or something.