I just changed my spark plugs on my '02 3.0 at 70k-- huge difference! After STF I picked up some NGKBKR6EQUP at my local auto parts store for $7.99/each. I had matched up the original plugs with the new ones are every mark/stamp is identical other than the BMW print on the plug. After spending $50 and 45min -- the X5 is running like a new car.

For those who are looking for what tools you need:

5/8 spark plug socket -- Sears has one for $5
socket wrench
6 inch socket wrench extension
10mm socket for the hex bolts
flat-head screw driver to help unlock metal bracket on coil
TLC / Patience

It is pretty easy to do but the furthest plug near the firewall gave me a bit of a challenge.

I wouldn't go 100k without changing the spark plugs. Since I will be keeping this X5 for awhile, I will be proactive and changing them every 25k-30k miles.

Next steps? Gonna change the oil (when my car hits 7k miles since BMW dealer changed it) before the regular intervals of the BMW maintenance.