basically they are just as good as copper, but can last longer. some people say the notice power with them, but that is usually from people who have platinums. i have yet to see a ngk copper user switch to iridiums and say they felt a gain in power.

another thing is people often mistake power gain from simply replacing old plugs with new ones. sometimes you also get the placebo effect also. which is when one thinks that since they spent $$$ on something makes them feel like they noticed a power gain.