Thanks guys, great info.
CBS - My performance gains were probably more from just delay of replacement than improvement over stock. I've heard plenty of good things about the iridiums though. I figure you change the plugs so rarely a few bucks is not a big deal and you save a ton by doing it yourself of course. Maybe I'm being taken for a ride spending more on them, but they work great and it isn't much more money.
One issue with iridium plugs is to be careful gapping them since the center electrode is so narrow and delicate. Easy to break, but I didn't have any issues.