I've had the E3 in my '00 XJ for 1 1/2 years now and no problems. I have the Bosch Platnium +4 in my '98 for two years.
Horsepower TV and Trucks have run dynos on both spark plug brands and have rated the E3 high on their performance rating.
No spark plug is going to give you magical MPG improvements. What they do do is to improve combustion burn resulting in gains in HP and torque. When you improve both of these with no other changes, MPG improvements can be had, but minimally. Gains would be over a period of time.