My F150 is coming up on 100,000 miles, which is the first service interval that calls for replacement plugs. I swapped them out this last weekend and the old plugs looked to be in excellent shape with the exception that the gap had increased from the .052" (?) spec to about .06. I was duly impressed.
This engine calls for 87 octane, and I haven't notice any pinging, so I'm thinking there was no extra wear from detonation. It's also got coil-over-plug doo-dads, which seemed to be in fine shape externally.
I have noticed the mileage decreasing a bit over the last few thousand miles; if the normal service doesn't bring it back up I'll have to dig deeper, possibly starting with the coil packs (since they're easy to get to
).