My only complaint with my KM is that the light metering is a bit off the wall. If you set the film speed "incorrectly" it pulls the metering back into line, the K1000 seems to be the same. Meanwhile the KX and K2 are spot on. I'm not sure if this is normal - I have seen others mention a need to adjust it this way.
I suspect that the difference is that IIRC the K1000 and KM are both average CdS metering systems whereas the K2 and KX are centre-weighted silicon cell systems. I can't say I've noticed any difference myself, though of course in most cases the need to open up or shut down a little compared to the indicated reading must always be watched out for.