I'll readily admit that it's really just stubborness and parochialism that keeps us using the English system. Back in the 70's the federal government tried to start a migration to the metric system, started making road signage in kilometers, etc., but people hated it.
I've lived about 5 years in Europe, and I'm in a science field, so I'm pretty saavy with the metric system. Also, the US military uses kilometers and meters. Thus, like Ben I'm kind of a mutt, but in a different way:
I never could get an intuitive grasp of Celsius temperatures, so I still think in Farenheit. I actually have to break out the formula and convert to make ANY sense of Celsius, beyond a few special situations: -40C, 0C, 37C, 100C.
I can do miles or kilometers with equal facility, as well as feet/yards and meters. Likewise, since I work in medicine I can do either pounds or kilograms. Or stone... :) Frankly, I PREFER the metric units, for all of the obvious reasons that have already been mentioned, but since US trail distances are always in miles I use that by default.
I saavy grams only by going through kilograms, though. I understand ounces much more intuitively. I know that this makes no sense, but there you are. I'm sure I'd easily catch the hang of grams if I had to, but in my field I'm much more likely to think 0.45kg than 450g, and for whatever reason one is more intuitive than the other. 450g sounds like almost nothing, but 0.45kg is almost a pound!
I'm also very "bilingual" in gallons/quarts/floz vs liters/mL. But like Ben I "think" about pack volume in liters- I have to convert from cubic inches if a manufacturer doesn't give volume in liters. Again I'm sure this is because of my field, and most Americans wouldn't know a milliliter from a tadpole.
For area? Acres are much more intuitive for me than hectares for large areas, but I can do either square feet or square meters- and a rough conversion between them is so unbelievably easy: a factor of ten! I grasp square inches better than square cm, though.
For energy/force I grok Joules/Newtons probably because of my science background. I probably couldn't even tell you what an erg is, though. For biological processes calories have pretty much taken over, so no conflict there. I never did grok Pascals, though. I do bar, millibar, atmospheres, psi.
So, maybe the US *is* converting slowly, if people like me are around. :)