I would like to use metric across the board, but, living in the US, I’m more or less conditioned to be more comfortable with Customary/Imperial/WTF you want to call those units for day to day use.
Sometimes the units are close enough in size that it doesn’t matter a heck of a lot, though. I mean, a meter is only a little bit longer than a yard – close enough for eyeballing and estimating. The same goes for quarts vs. liters (and by extension, gallons which equal 4 quarts). Centimeters, while not being particular close to any single alternative unit, is roughly the width of my pinky finger, so close enough for eyeballing and estimating again.
The metric units in common use that I don’t get intuitive senses of their scale are the kilometer, the kilogram, and degrees Celsius.
Let’s leave out the more scientific ones like the weber, tesla, coulomb, etc. I’m not even sure they have equivalents in US Customary.
Oh, and then there’s the whole MKS vs cgs thing… but that’s yet another debate.
Now I’m having flashbacks to my college electricity and magnetism classes…