Halon is perfect for car fires, which are almost always electrical or flammable liquid in nature. Very expensive, though. The only times I've seen Halon in an application have been in server rooms with lots of sensitive electronics.
Yeah, they're expensive- but a heckuvalot cheaper than a burndown. ABCs are cheaper, but messier- and you gotta remember to maintain them like Righty Tighty said. Otherwise you'll just be blowin' propellant.
On a side note, years ago we were sittin' at a car show having a couple of cold ones when a 442 driving by suddenly died and started smoking... he pops the hood and it goes "Whoof!". We ran up and put it out with the last of our beers. Got towed away; then a couple hours later the guy shows up and plops a fresh case down in front of my car, saying "You guys saved my car. This is the least I can do." Only problem is his car smelled like a dive bar now- smoke and stale beer. Better than the alternative, by far.