I had them on my Lexus SC for 30,000 miles, worked fine, but nothing special. I switched over to Denso Iridiums, again works fine, but didn't notice any extra benefits. Bottom line is if the car is well maintained to begin with, changing spark plugs will not yield any noticable performance difference.
I have never had a vehicle run better on these plugs. I have tried them twice in two different vehicles and pulled them out because they ran worse and got a refund. NGK or Nippon Denso make the best plugs, IMHO. The factory stuff is really optimized, in most cases.