Well, if you have been following the saga of the shorting spark plug here and here you'll remember that the high performance spark plugs I got from Enerplus (called Pulstar) shorted and burned out my ignition coil, but the company reimbursed me for the plugs and the coil replacement. My mechanic put stock plugs back into the 2002 Prius, and since then I've been sadly watching as the gas mileage deteriorated by 0.1 mpg per month. And pushing on the steering wheel and saying "come on, come on..." as the car struggled onto the freeway in the morning.
Anyway, I finally broke down a couple weeks ago and bought another set of Pulstar plugs. Enerplus has been working on improving them, and version 2.0 supposedly has a longer lifetime as well as a fix for the occasional high resistance problem that plagued the original plugs I had. Plus they are less expensive.
I recently had the car in for a 75,000 mile checkup and asked my mechanic to put them in again. He looked at me a bit funny and asked if I was sure, but when I told him that the company had made good on their guarantee, he was relieved. I've reset the mileage, hopefully it will get back up to 46 mpg it was before the resistance started creeping up last summer.