The iridium ones have a very long expected service life, but this can mean as the electrode wears away, that the gap gets wider, leading to a reduction in ignition efficiency. This long service life can also make removing the plugs difficult as they are not being removed regularly, but this is being a little petty perhaps? :think:
You'd be better of using a standard type plug such as the NGK BPR6ES which you change every main service really, but it's down to personal opinion and choice. For the saving you make buying the standard plug, you could also buy a new set of HT leads at the same time.
Just removed NGK iridium after many moons have passed and not had any issues at all in fact I had forgotten what I had installed untill I dug themout.Anyway engine is on standard NGK now but will deffinately put iridium back in at the next major service.