I've seesawed on this over the last few years.

I. The Good

I was originally* a big fan, faster than gas stations, but still compatible with at home "pumping" for city driving. It's basically a propane refill model, shops could buy back your old battery while they sold you a new one, so eventually you're really just paying for the energy at basically no wait.

II. The Bad

I grew more pessimistic as I started shopping for aftermarket cell phone batteries, noticing that:

a) Batteries degrade over time (in terms of capacity or depth of discharge)

b) Batteries are expensive, especially car batteries (though this should improve over time)

c) It's impossible to really know the exact capacity of a battery without a power cycle or two

Would you be willing to swap phone batteries with a stranger? What if the battery cost $4000? Who eats the cost of lemon batteries on the market? Does a sort of Gresham's Law kick in where suddenly everyone's dumping a lemon in every transaction?

Buying gas involves some trust, trust that they're not loading it up with crud, that it really has the energy capacity it says it does, that it's not 30% ethanol instead of 0, 10, 15. But I believe, based on the factors above, the trust relationship with a battery refill is a little more intense.

III. The Beautiful?

That said, now I'm of course swinging optimistic again. Car and Driver taught us that Tesla's diagnostic tools are awesome. Serial individual batteries, let the owner see its entire history, hidden lemon problem basically solved. Maybe build some "insurance" into battery costs, guarantee certain lifespan, smoothe out the cost of duds.

Musk can dodge some of the authentication problems by just keeping a tight grip on the infrastructure and the technology, but for this to really take off, Musk will want to allow competition, standardize battery swapping tech, encourage mom and pop "battery stations" to quickly build out the infrastructure in all the small towns he doesn't know are on the map. Not sure if that's in his personality though...

* Originally = Few years back I sat on a few panels for energy and transportation, wrote some white papers, nothing big.

EDIT: formatting, headings