Glow Plugs

Do glow plugs wear out like a spark plug or just go out like a light?
To rephrase can a glow plug still have continuity but be less effective
than a new one?

A glow-plug is just a heating element, it works just like the heating
element in your hot water heater.
they go bad the same way too, all at once.
are you having a problem with your glow plugs, or are you thing about
routine maintence?

I live in a mild climate near Seattle so the need for them is light. I
was just wondering because if I did have 1 or more go bad I would just
replace them all if they wear. If they are work or don't work I would
probably just replace the ones that didn't work. I have just noticed a
couple of times that my truck did not start as easy as it normally
does. Thanks for the answer.