I grew up in the UK which switched from pounds, shillings and pence to "metric" money while I was in elementry school. By the time I went to college everything was taught in the metric (SI) system, which for most engineering calculations is a lot simpler to use for me.
Of course, by the time I got out into the big world, everywhere I worked things were measured in the old British system. So now, on the rare occaision that I actually have to make a calculation, the first thing I do (after looking up the formula - CRS) is convert everything to metric, do my calculation, then convert the answer back.
On the subject of metric in the US, something that irks me. I think it is probably true to say that greater than 50% of the goods sold in the US today use metric fasteners. Why then, when you go to your local hardware store for a bolt or something, is there three rows of standard fasteners and just one lttle box of metric stuck on a top shelf somewhere?
On a lighter note I finally got my new speedometer cable. My 100% accurate 53 year old Smith's Chronometric speedometer clocked the old girl (now with close to 4000 MILES on her rebuilt engine) at 75 one day. (OK, so it was downhill with the wind behind).