Even really bright people screw up when metric and non-metric teams have to cooperate -- like the famous case of the Mars Climate Orbiter that failed to orbit.
http://en.wikipedia.org/wiki/Mars_Climate_Orbiter
I'm most comfortable with US measures, but I have a few "approximations" I can use when I have to. And bottled beverages are sold in metric sizes here, so I'm pretty good with them.