You can swap a new battery into your Tesla Model S in 90 seconds. Here's the proof. No details yet as to how much they'll charge for a battery swap. It sounds like if you keep the new battery you pay the difference, perhaps charged based on battery age and capacity differences.
I have done one long road trip so far, from San Francisco to Los Angeles and back, and I stopped at all 3 supercharging stations between the two cities in each direction. It added about a two hours to my trip in each direction because the first supercharging station was packed on the way down, and I couldn't find an open station to charge at in Los Angeles before hitting the road on the way home.
The next time I make the road trip, I hope to try the battery swap, not just to satisfy my curiosity, but to save on time. Since that trip they've also issued a software update for the car that now shows charging stations you've visited and Tesla supercharging stations on the onboard maps.
As with owning something like an iPhone, it's always fun when the product improves over time. Usually it's because of software updates, but a battery swap is really a hardware upgrade.
I guess this obviates the need for someone like Mophie to make a $5,000, 250-pound external battery pack for the Tesla.