When I was living in South America I met hundreds of backpackers from all over the world that were forced to go through the US on their way South. Many of them had hilarious, stupid and scary stories of how customs treated them when they only had a ~2 hour layover and had no interest in the US what-so-ever. They all vowed to never fly through the US again, even though it will result in higher flight costs.
Both times they didn't have enough staff to man the separate customs desk, and ran us through a general customs hall.
I prefer via Dubai, Hong Kong, or Shanghai. Border control officers there, while still clearly doing their job meticulously, manage to be polite as well.
The idea is that you "enter the country" when going out from the main part of the airport, but you can go from any arriving flight to any departing flight without doing so, and most of the facilities - shops, waiting rooms, etc - are 'inside the zone' as well.