I think they're likely profitable in the cities where they've achieved scale. Specifically, I suspect that Uber (and Uber drivers) in the Boston/Cambridge market (where I live) are making a net profit overall.

Drivers who drive full time can afford to pay for the car expenses (variable and fixed). Drivers who drive part time are able to cover the variable costs (and they already had their fixed costs anyway). Uber is probably not losing money in Boston. If all three of those things are true, Uber is sustainable in Boston.