You can get to a point where a used Model S does start to become cheaper than a vaguely equivalent normal larger commuter car such as a BMW x30d/Mercedes x350 (I run an E350) - you need to keep piling the miles on though, as it is the fuel duty which pays for the electric car.
The other issue for me is the risk of it failing. This is one that i struggle with. The car (a Model S) is totally unique in its drivetrain and I have absolutely no idea where i would even start with having a faulty one repaired. Unfortunately I think that the whole thing could well end in disaster, once the mileage really racked up on it, and unfortunately racking up huge mileage is the entire point in the thing.
Backdrop: I am absolutely not a risk averse person, i don't have any insurance or insured products in my life other than mandatory basic car insurance with a £1500 excess, and i run a couple of large used diesels out of warranty over significant mileages. The difference is though, that every garage can fix a diesel, and even in a worst case scenario where the car becomes too much to deal with, the investment in it is not great.