The clear plastic can crack, but the metal ones are like the ever ready bunny. Once the coil gets really grungy, you can soak it in alcohol or hot water, then even dry burn it, you can see the coil, and if you're careful .... it's baaaaaack! Some have given their lives for the cause, but we have some we keep loading up with different flavors and swapping with my wife that we've had for months. Their weakness is a tendency to wick poorly if the wicks are compressed. Most cognoscenti remove both the washer shaped insert AND the cup-shaped insert (careful, with hooked needle), then gently pulled up the wick on either side of the cup, just a hair to loosen the glass fiber) then replaced the cup-shaped insert LOOSELY. That gives you excellent wicking. The best way to fill is with an 18 gauge needle right into the tiny fill holes in the cup-shaped insert, or remove that, drip down the sides, then replace it. You'll see. Look on YouTube for tutorials. Worth the extra work. Or else just use as is and toss them if/when they don't wick well.
Oh, and the plastic ones, once they crack, you can use the innards in an improvised tank.