Twist and Pull... they're boogers too get off after being on for a while. Just the way they are, most likely nothing "wrong" with them if your not showing a misfire or a check engine light.
They factory AC Delcos claim a 100,000 mile life expectancy (assuming its driven easily for that 100k) most last 80k(ish) that I've seen. Really depends on the driver and the car maintenance.