Before reading the many posts in many car forums, I thought I'd do the 2004 Mazda3 a favor and replace the plugs. I installed a set of Bosch Fusion plugs and immediately had a subtle miss in the transition region while cruising. I reinstalled the original plugs after checking the gap and the engine now runs flawlessly.

IMO, these things are a gimmicky and defective design and a huge number of people have had trouble with 'em. I'm amazed that they haven't been taken off the market and the company forced to do a full recall. There was a mail-in rebate that I already sent in, so I can't even return 'em. Avoid. Avoid. Avoid.