I just replaced mine. The plugs looked perfect (no erosion, tan color), and the car has 82k miles on it. I did a bit of reading on platinum plugs, and even though the manufacturers often recommend replacement at 100k miles, lots of people say the plugs degrade in performance by 50k miles. If you do decide to replace, I'd recommend going with the Denso Iridium spark plug.
http://www.denso.co.jp/PLUG/iridium-e/
http://www.densoiridium.com/
The NGK Iridium plug looks fine too. In fact, it looks the same as the Denso Iridium plug.
The plugs are a PITA to replace. It will probably take you several hours to do it the first time if your engine is clean to start with. If its not clean, I recommend degreasing around the spark plugs before starting. BTW, I ended up breaking all the little plastic hangers for the lead wires.
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Mychailo
:: 2006 Silver Mitsubishi Evolution 9, E85, 34 psi peak, 425wtq/505whp DJ ::
1995 Laguna Blue SVX L AWD 5MT (sold)
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SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads.
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