[embee]

You won't notice any difference in power or economy, assuming it's actually running properly anyway.

What they offer is improved initiation of the burn and lower voltage requirements from the rest of the ignition system. In simple terms it will start more easily and run more consistently. The differences are subtle, you may or may not notice anything. They also help when the fuelling is less than perfect, for example carb engines cold starting and running off choke sooner, or tip-in after overrun when fuelling tends to struggle.

They do also last somewhat longer than the traditional design, but again that may not actually be much of an issue to you.

Personally I use iridium plugs wherever possible, it's relatively little oncost in the big scheme of things (e.g. fuel, tyres, insurance, road tax etc) and as such there's not much reason not to fit them.

Be aware there are reportedly some fake NGK iridium plugs about, so buy from a reputable source.