i changed mine with ngk iridium. (lucky to have them under $30 coz no one outbid me from ebay) honestly, i can't feel any difference whatsoever...maybe it generates better mpg or retrive little hps...but i just don't feel it...
my friend with ford focus changes plugs every 10k miles coz he uses cheapest (like $1.19/ea from walmart) plugs...and he said it's kinda wate money coz the factory manual suggest replacement every 60k miles...however, for total less than 5 bucks and easy installation...he keep doing so...and keep the overall mpg and performance constant before plugs build up too much carbon...
