When is it time to get a new U-Lock? Is there ever a time?

I've been blessed and have never had a problem with bike theft. I have an OnGuard Mini U-Lock and that's it. I've had it for about two years, and it's been used nearly every day.

I was in San Francisco, then. Now I'm in NYC. I'm considering buying the Fahgettaboudit U-Lock from Kryptonite. I'm not looking to buy the chain.

I know that the Kryptonite U-Lock is better than the OnGuard Mini, but my question is about the wear and tear of a u-lock? Would it be wise to get a new one?