As an easy to use system for measuring distance, weight or volume the metric system is far superior.
But it's still hard to visualize how "much" something is since we're used to doing that in English and need to convert to more comfortable units. 15 Km - is that a long way? 100 kph - that's really fast, isn't it? Temp is 12 C - hmm, is that hot or cold? If something is 30 cm, that must be pretty long, right? (No, it's just about one foot) About the only equivalent I'm comfortable with is a liter since it's just about one quart.
It's mostly a matter of unfamiliarity and is a lot like speaking a foreign language. If you have to think of what you want to say in English and then translate it, that's difficult. Once you learn to think of what you want to say in the other language right off it becomes a lot easier.
