the originals in my 4.6is were bosch. I replaced with NGK. Noticed zero difference. I replaced at 105K.

I've been running the NGK Iridium IX plugs for about 35K miles now. No issues. No real power gain over stock except it was cheaper and is supposedly good for 100K miles. Iridium is also supposed to burn hotter or assist in a more complete burn over platinum for the same given voltage.

An NGK rep told me the aftermarket Iridiums are good for about 40,000 - 50,000 miles. They only last about half as long as the OE Bosch or NGK plugs. That is why the OE plug is typically twice as expensive. It lasts twice as long.