On my 2001 job 2 ST I changed them at 45k miles and they were shot. Big difference after I changed. Went with the required platinum plugs. I know the manual says 100k but that is crazy, listen to the manual and what the dealer says and you will be dealing with horrible performance. Just my opinion.
Now I have over 6700 miles on my '07 ST and in the summer I will be pulling a plug to see what they look like and what the gap is set at.
Read my spark plug project in the Projects area and you can see pics of the burnt plugs. Also read that each plug was gapped differently from the factory and some of the way out of specs. I guess quality is not always job one.