We live in the Bay Area and normally don't take taxis too often, but had a recent outing where we took a BART train into SF and exited at one of the Mission St. stations. Our destination was a mile or so from that, and we decided not to rely on a bus for the last mile, as we had a reservation to meet.
We downloaded the Uber app just before this, not knowing how prevalent cabs would be in the vicinity. We got out of the station, and saw no cabs for 5-10 minutes, nor any taxi stand there. So we hailed an Uber using the app.
The Uber map showed several cars in the vicinity, and one showed up in about 5 minutes, telling us the point along the street to wait, car type, and license number. The first ride after downloading the app was free up to $15, and we took a very smooth return trip to BART on Uber also. Trip and tip was charged to our credit card, and no cash tip needed. Price was similar to cab on the paid return trip. It worked well for us.
If you're in popular tourist areas like the Embarcadero or Union Square, cabs are easier to catch than outer areas. But it's good to have the Uber or similar alternative app in case. Just beware of flex pricing, as they may raise their rates in times of high demand.