The covers work for me, I never walk more than a few steps without them on and this means I get at least 9 months out of a set of keo cleats. I'm running a set of grip cleats on my winter shoes and a set of standard cleats on my summer/race shoes at the moment - both have lasted since last october when I replaced them.
The grip cleats do seem to be able to take more walking about, but they are a bit annoying as they can catch the pedal when clipping in, so aren't as slick to use.