It's more expensive than the train, but cheaper than buying a car, paying operating expenses, paying for parking at my apartment and paying for parking at work by a dramatic amount.
For longer uses or multi-stop errands, I grab a Car2Go or a Zipcar. For whole-day trips or longer, I head to National where I can pick any car from the Aisle for around $50/day.
Same argument could be applied to car rental businesses (some of which are owned by automakers which makes the relationship super-cozy), yet I've never seen a deal from Avis or Hertz to make it a worthwhile replacement to owning.
Big cities have an added cost of parking and extra miles driven to find a gas station or a public charger. But big cities also have low-cost public transportation (or so I've heard, I live in LA).
In smaller towns the cost of ownership tends to be lower and with sub-$10,000 products on the market http://jalopnik.com/5072303/new-pricing-of-9990-makes-2009-n... you have a reliable guaranteed ride whose cost is usually spread over 6-7 years.
Being able to book one to use for a journey then let it go home would be great.
Important to note that currently it's the Uber and Lyft drivers that are subsidizing the cost of getting the car to you - they are not getting paid for the miles driven. Whether or not this cost will be subsidized by Uber/Lyft in the future remains to be seen.
If there is no driver involved the cost structure for you and Uber is pretty much the same. Except Uber can probably negotiate better deals on new cars and service (due to volume) and also achieve higher utilization. On the other hand, Uber of course needs to make a profit.