I moved to the US from a country using metric system and different formats for things like date and currency. I don't really find the difference important at all and have no trouble switching between units and formats (my job requires me to exchange information with teams in Europe and Canada). I did find that for the sake of avoiding misunderstandings date should always start with the year. It's also good for sorting files and folders. Daylight savings time is the only thing I wish we didn't have in the US.
I have lived in English and Metric countries and have had no trouble with formats or units. I think it is because I didn't try to convert anything, just learned it as a new system. My wife on the other hand had a bunch of mental trick to convert between Metric and English, but she also never had any trouble with date or time formats.
The biggest problem to me is time zones and summer/winter time. I wish we all used UTC.
