For 99% of everyday use, imperial. For anything involving calculation, or for engine displacement, metric. In all seriousness, it's self-evident that metric is far better, but my head is too far up my imperial ass to want to develop an intuitive feel for metric (we have another 125 kilometers to drive; um...). I'm not alone, hence we still use imperial.
Personally, I have an intuitive understanding of both systems. However, metric is superior for any serious application. The ratios between Imperial units are incredibly cumbersome from a math perspective, and if I'm doing any sort of calculation, I'd rather not have to have an extra non-trivial conversion step (e.g. 8 8ths/inch, 12 in/foot, 3 ft/yard, 5280 ft/mile, with non-distance measurements being even worse. What kind of a numbnuts retard came up with that?). That being said, if you rednecks want to keep measuring your distances according to the size of some medieval guy's feet, or your weights according to the displacement generated by some guy's left nut, be my guests. Now that's what I'm talking about. Kelvins for everyone! Now I'm gonna leave for a couple of minutes and when I come back, this room better be at "SATP" or I'm gonna pop a cap in someone's ass.
Regarding learning a new system In the mid '70's, my family and I lived for five years in West Germany. In the first year, I was converting kms to miles using 1.6 and .625. By the second year I was thinking in metric and had no need to convert. However, I never could get used to Celsius, still can't.
Anything that requires me to have an actual feel for what's going on, ("I want a pound of pastrami") I use Imperial. Anything else, I'm calculating in metric. I DO find it interesting to note that in pretty much all manufacturing and engineering, everything is done in metric, even when the consumer goods are often marked in Imperial measurements. Looking at a car, the speedometer's markings that show miles per hour are laid out on a schematic in terms of millimeters.
For engineer brain - having everything in obsessive base 10 configuration makes life WAY easier. The mental math is much more intuitive than imperial once you get used to it. I can work pretty easily in imperial or metric - and I know I'm much more likely to tell someone that I'm 6'1 then that I'm 185.4 cm tall - but anything where I have to do a bunch of math with the numbers - it's just much simpler to work with metric.
Try having an old man that says mile whether he means miles or kilometres. Then gets cranky when you want to confirm it. Last time I ever ask him for directions....