I've used the Speedplay X's for years. I go through at least two set of cleats a year. I figure I go through cleats twice as fast on a brakeless fixed gear vs. geared.

On the Speedplays the "wires/springs" flatten out. They get tough to get in and out of and then start to release early. I have even broken the "wire/spring" on a set. It sucks when you are pulling up on the pedals to stop and you here a snap and your knee flys up in the air!

I have learned the signs of wear and replace my cleats before they fail on me.