I use Uber when I'm in SF. It seems like a very San Francisco specific service. I would never use it in NYC because I've never had problems getting a cab and the cabs already take credit cards. I would also not use it in any other city I've been in the USA because they are so spread out I always have to have my own car, anyway.
Uber mostly seems like a better way to hail a cab in SF because hailing a cab there was previously next to impossible.
I am not sure how relevant it is in other cities but it will be interesting to see how they do.
If you think the only city that is difficult to hail a cab in is SF, you are sadly mistaken. I can personally say Austin, Atlanta, Baton Rouge, New Orleans, Athens, etc. desperately need a better way to hail a cab.
Uber mostly seems like a better way to hail a cab in SF because hailing a cab there was previously next to impossible.
I am not sure how relevant it is in other cities but it will be interesting to see how they do.