Convenience:
* There's a very easy-to-use app to request a driver that shows the locations of nearby cars and an estimate on how long it'll take for a car to arrive. No need to try and flag down a cab or call a company.
* Payment is done via stored credit card info, so no need to worry about paying at the end of the ride. No need to worry about tip, either.
Comfort:
* The town car is far more comfortable than a normal taxi. Nice seats, ample leg room (and I'm a large guy), no ads, actually useful air conditioning.
* There are magazines to read, if that's your thing. Some of the cars seem to be wifi equipped (one of my drivers has told me that that car had wifi, at least).
Experience
* They definitely ride the line between limo and taxi, to the point where I can't help but laugh. The driver even hops out of the car to open the door for you!
Really, if you're in SF, it's worth checking out Uber at least once.