I'd agree with the others. I've seen dyno testing showing some plugs are better, but compared to what? The OEM plug thats been on the car for 50k miles? I think any new plugs will be better than old worn plugs.

I tested (got 'em free) some Pulstar plugs a while ago. No noticeable difference in power or mileage. One also went bad after approximately 1 year, so I threw my old plugs back in.