I saw these a while back in popular science and decided against them...
Some interesting discussion
here.
And
here is about the closest thing to scientific test results I could find.
Assuming I could get a 2.7% increase in mileage as described in the test above, I would save about $40 per year over 6000 miles of driving. So the break even point for $200 worth of plugs would be about 5 years or 30k miles of driving. Not worth it in my opinion, especially since I doubt I would even see a 2.7% difference.