Uber has been running UberPool promotions in San Francisco for almost two weeks now and UberPool rides to San Francisco Airport are just $15. I took them a couple of times and was pooled every time with one other rider.
This price beats SuperShuttle, which is $17 from most areas in San Francisco. SuperShuttle rides usually pick up 5-6 different riders and you often end up spending 60+ minutes in the shuttle.
Uber clearly wins as long as prices stay in the $15 range.
Now New York City has joined in and Uber caps the UberPool rides at $36 from JFK. The SuperShuttle rides are $20, so Uber is much more expensive, but again the service and the car you get will likely be much better in an Uber.
It's astonishing how quickly Uber has changed from a great (but extremely expensive) private limo service to an effective contender of public transportation. The company enjoys network effects almost as much as Google did when it first started out.