It is huge in NY where most people don't have cars. I've had mostly good experiences and I'm thankful for it. I like that I don't have to pay in cash or deal with tips (charges your account).
But in peak times they often charge surge pricing which costs way more than a taxi/car service, and sometimes if there is traffic in the city it takes them a long time to come (more than estimated). I wish they had it when I lived in LA so I could go out and not worry about driving!
Have you tried it? What has been your experience?