Fresh shoe cleats ROCK!
I have an older pair of cycling shoes with SPD cleats that I've used extensively. Lately I've noticed that I've had more difficulty clipping in and out on a couple of my bikes. I had examined the cleats visually and didn't think they could have been worn very badly because I could still read most of the writing on the cleat surface.
Well, today I popped on a fresh pair of cleats and tried them out. ZOWIE! I couldn't believe how much difference the new cleats made.
I would expect more of the wear to be on the edges/ends of the cleats rather than the surface... after all, when clipped in, it is the ends that are holding your feet in place, not the face of the cleats.