My guess is that the confustion isn't so much the system (10 mm to 1 cm, 100 cm to one m, etc), but the reference. We can all look at something and say, "thats about 18 inches", but its much harder to say, "thats like 33 cm". Same thing with miles/km. We all can estimate rather easily what a mile or two is, but we have no frame of reference for km.
That's my problem with metric, anyway.